diff --git a/app/models/PullRequest.java b/app/models/PullRequest.java
index edba150c8..3fb5527bc 100644
--- a/app/models/PullRequest.java
+++ b/app/models/PullRequest.java
@@ -149,7 +149,7 @@ public class PullRequest extends Model implements ResourceConvertible {
         joinColumns = @JoinColumn(name = "pull_request_id"),
         inverseJoinColumns = @JoinColumn(name = "user_id")
     )
-    public List<User> reviewers = new ArrayList<>();
+    public Set<User> reviewers = new HashSet<>();
 
     @OneToMany(mappedBy = "pullRequest")
     public List<CommentThread> commentThreads = new ArrayList<>();
@@ -999,7 +999,7 @@ public static void changeStateToClosed() {
     }
 
     public void clearReviewers() {
-        this.reviewers = new ArrayList<>();
+        this.reviewers = new HashSet<>();
         this.update();
     }
 
@@ -1008,8 +1008,9 @@ public int getRequiredReviewerCount() {
     }
 
     public void addReviewer(User user) {
-        this.reviewers.add(user);
-        this.update();
+        if(this.reviewers.add(user)) {
+            this.update();
+        }
     }
 
     public void removeReviewer(User user) {
diff --git a/app/models/User.java b/app/models/User.java
index df98e7af5..fe30ae60b 100644
--- a/app/models/User.java
+++ b/app/models/User.java
@@ -660,4 +660,24 @@ public void createOrganization(Organization organization) {
     private void add(OrganizationUser ou) {
         this.organizationUsers.add(ou);
     }
+
+    @Override
+    public boolean equals(Object o) {
+        if(!(o instanceof User)) {
+            return false;
+        }
+        if(o == this) {
+            return true;
+        }
+        User user = (User) o;
+        return this.id.equals(user.id) && this.loginId.equals(user.loginId);
+    }
+
+    @Override
+    public int hashCode() {
+      int result = 31;
+      result = result * 37 + this.id.hashCode();
+      result = result * 37 + this.loginId.hashCode();
+      return result;
+    }
 }