diff --git a/src/librustdoc/clean/cfg.rs b/src/librustdoc/clean/cfg.rs
index 13df1892a5f17..84e6ff648a38f 100644
--- a/src/librustdoc/clean/cfg.rs
+++ b/src/librustdoc/clean/cfg.rs
@@ -202,23 +202,34 @@ impl ops::Not for Cfg {
 
 impl ops::BitAndAssign for Cfg {
     fn bitand_assign(&mut self, other: Cfg) {
-        if *self == other {
-            return;
-        }
         match (self, other) {
             (&mut Cfg::False, _) | (_, Cfg::True) => {}
             (s, Cfg::False) => *s = Cfg::False,
             (s @ &mut Cfg::True, b) => *s = b,
-            (&mut Cfg::All(ref mut a), Cfg::All(ref mut b)) => a.append(b),
-            (&mut Cfg::All(ref mut a), ref mut b) => a.push(mem::replace(b, Cfg::True)),
+            (&mut Cfg::All(ref mut a), Cfg::All(ref mut b)) => {
+                for c in b.drain(..) {
+                    if !a.contains(&c) {
+                        a.push(c);
+                    }
+                }
+            }
+            (&mut Cfg::All(ref mut a), ref mut b) => {
+                if !a.contains(b) {
+                    a.push(mem::replace(b, Cfg::True));
+                }
+            }
             (s, Cfg::All(mut a)) => {
                 let b = mem::replace(s, Cfg::True);
-                a.push(b);
+                if !a.contains(&b) {
+                    a.push(b);
+                }
                 *s = Cfg::All(a);
             }
             (s, b) => {
-                let a = mem::replace(s, Cfg::True);
-                *s = Cfg::All(vec![a, b]);
+                if *s != b {
+                    let a = mem::replace(s, Cfg::True);
+                    *s = Cfg::All(vec![a, b]);
+                }
             }
         }
     }
@@ -234,23 +245,34 @@ impl ops::BitAnd for Cfg {
 
 impl ops::BitOrAssign for Cfg {
     fn bitor_assign(&mut self, other: Cfg) {
-        if *self == other {
-            return;
-        }
         match (self, other) {
             (&mut Cfg::True, _) | (_, Cfg::False) => {}
             (s, Cfg::True) => *s = Cfg::True,
             (s @ &mut Cfg::False, b) => *s = b,
-            (&mut Cfg::Any(ref mut a), Cfg::Any(ref mut b)) => a.append(b),
-            (&mut Cfg::Any(ref mut a), ref mut b) => a.push(mem::replace(b, Cfg::True)),
+            (&mut Cfg::Any(ref mut a), Cfg::Any(ref mut b)) => {
+                for c in b.drain(..) {
+                    if !a.contains(&c) {
+                        a.push(c);
+                    }
+                }
+            }
+            (&mut Cfg::Any(ref mut a), ref mut b) => {
+                if !a.contains(b) {
+                    a.push(mem::replace(b, Cfg::True));
+                }
+            }
             (s, Cfg::Any(mut a)) => {
                 let b = mem::replace(s, Cfg::True);
-                a.push(b);
+                if !a.contains(&b) {
+                    a.push(b);
+                }
                 *s = Cfg::Any(a);
             }
             (s, b) => {
-                let a = mem::replace(s, Cfg::True);
-                *s = Cfg::Any(vec![a, b]);
+                if *s != b {
+                    let a = mem::replace(s, Cfg::True);
+                    *s = Cfg::Any(vec![a, b]);
+                }
             }
         }
     }
diff --git a/src/librustdoc/clean/cfg/tests.rs b/src/librustdoc/clean/cfg/tests.rs
index 309f72040615d..d090bf325038f 100644
--- a/src/librustdoc/clean/cfg/tests.rs
+++ b/src/librustdoc/clean/cfg/tests.rs
@@ -87,6 +87,12 @@ fn test_cfg_and() {
         x &= word_cfg("test3");
         assert_eq!(x, word_cfg("test3"));
 
+        x &= word_cfg("test3");
+        assert_eq!(x, word_cfg("test3"));
+
+        x &= word_cfg("test4");
+        assert_eq!(x, Cfg::All(vec![word_cfg("test3"), word_cfg("test4")]));
+
         x &= word_cfg("test4");
         assert_eq!(x, Cfg::All(vec![word_cfg("test3"), word_cfg("test4")]));
 
@@ -105,6 +111,18 @@ fn test_cfg_and() {
             ])
         );
 
+        x &= Cfg::All(vec![word_cfg("test6"), word_cfg("test7")]);
+        assert_eq!(
+            x,
+            Cfg::All(vec![
+                word_cfg("test3"),
+                word_cfg("test4"),
+                word_cfg("test5"),
+                word_cfg("test6"),
+                word_cfg("test7"),
+            ])
+        );
+
         let mut y = Cfg::Any(vec![word_cfg("a"), word_cfg("b")]);
         y &= x;
         assert_eq!(
@@ -119,6 +137,14 @@ fn test_cfg_and() {
             ])
         );
 
+        let mut z = word_cfg("test8");
+        z &= Cfg::All(vec![word_cfg("test9"), word_cfg("test10")]);
+        assert_eq!(z, Cfg::All(vec![word_cfg("test9"), word_cfg("test10"), word_cfg("test8")]));
+
+        let mut z = word_cfg("test11");
+        z &= Cfg::All(vec![word_cfg("test11"), word_cfg("test12")]);
+        assert_eq!(z, Cfg::All(vec![word_cfg("test11"), word_cfg("test12")]));
+
         assert_eq!(
             word_cfg("a") & word_cfg("b") & word_cfg("c"),
             Cfg::All(vec![word_cfg("a"), word_cfg("b"), word_cfg("c")])
@@ -145,6 +171,12 @@ fn test_cfg_or() {
         x |= word_cfg("test3");
         assert_eq!(x, word_cfg("test3"));
 
+        x |= word_cfg("test3");
+        assert_eq!(x, word_cfg("test3"));
+
+        x |= word_cfg("test4");
+        assert_eq!(x, Cfg::Any(vec![word_cfg("test3"), word_cfg("test4")]));
+
         x |= word_cfg("test4");
         assert_eq!(x, Cfg::Any(vec![word_cfg("test3"), word_cfg("test4")]));
 
@@ -163,6 +195,18 @@ fn test_cfg_or() {
             ])
         );
 
+        x |= Cfg::Any(vec![word_cfg("test6"), word_cfg("test7")]);
+        assert_eq!(
+            x,
+            Cfg::Any(vec![
+                word_cfg("test3"),
+                word_cfg("test4"),
+                word_cfg("test5"),
+                word_cfg("test6"),
+                word_cfg("test7"),
+            ])
+        );
+
         let mut y = Cfg::All(vec![word_cfg("a"), word_cfg("b")]);
         y |= x;
         assert_eq!(
@@ -177,6 +221,14 @@ fn test_cfg_or() {
             ])
         );
 
+        let mut z = word_cfg("test8");
+        z |= Cfg::Any(vec![word_cfg("test9"), word_cfg("test10")]);
+        assert_eq!(z, Cfg::Any(vec![word_cfg("test9"), word_cfg("test10"), word_cfg("test8")]));
+
+        let mut z = word_cfg("test11");
+        z |= Cfg::Any(vec![word_cfg("test11"), word_cfg("test12")]);
+        assert_eq!(z, Cfg::Any(vec![word_cfg("test11"), word_cfg("test12")]));
+
         assert_eq!(
             word_cfg("a") | word_cfg("b") | word_cfg("c"),
             Cfg::Any(vec![word_cfg("a"), word_cfg("b"), word_cfg("c")])
diff --git a/src/test/rustdoc/duplicate-cfg.rs b/src/test/rustdoc/duplicate-cfg.rs
index 505d6ee769ab3..9ccc5d7882eb8 100644
--- a/src/test/rustdoc/duplicate-cfg.rs
+++ b/src/test/rustdoc/duplicate-cfg.rs
@@ -1,15 +1,42 @@
+// ignore-tidy-linelength
+
 #![crate_name = "foo"]
 #![feature(doc_cfg)]
 
-// @has 'foo/index.html'
-// @!has '-' '//*[@class="stab portability"]' 'feature="sync" and'
-// @has '-' '//*[@class="stab portability"]' 'feature="sync"'
+// @has 'foo/struct.Foo.html'
+// @has '-' '//*[@class="stab portability"]' 'This is supported on feature="sync" only.'
 #[doc(cfg(feature = "sync"))]
 #[doc(cfg(feature = "sync"))]
 pub struct Foo;
 
+// @has 'foo/bar/struct.Bar.html'
+// @has '-' '//*[@class="stab portability"]' 'This is supported on feature="sync" only.'
 #[doc(cfg(feature = "sync"))]
 pub mod bar {
     #[doc(cfg(feature = "sync"))]
     pub struct Bar;
 }
+
+// @has 'foo/baz/struct.Baz.html'
+// @has '-' '//*[@class="stab portability"]' 'This is supported on feature="sync" and feature="send" only.'
+#[doc(cfg(all(feature = "sync", feature = "send")))]
+pub mod baz {
+    #[doc(cfg(feature = "sync"))]
+    pub struct Baz;
+}
+
+// @has 'foo/qux/struct.Qux.html'
+// @has '-' '//*[@class="stab portability"]' 'This is supported on feature="sync" and feature="send" only.'
+#[doc(cfg(feature = "sync"))]
+pub mod qux {
+    #[doc(cfg(all(feature = "sync", feature = "send")))]
+    pub struct Qux;
+}
+
+// @has 'foo/quux/struct.Quux.html'
+// @has '-' '//*[@class="stab portability"]' 'This is supported on feature="sync" and feature="send" and foo and bar only.'
+#[doc(cfg(all(feature = "sync", feature = "send", foo)))]
+pub mod quux {
+    #[doc(cfg(all(feature = "send", feature = "sync", bar)))]
+    pub struct Quux;
+}