From fc30f7c9742c85c7e88589c9b62186dd475a1c38 Mon Sep 17 00:00:00 2001
From: Elliot Coyle <elliotcoyle@ekmsystems.co.uk>
Date: Fri, 8 Nov 2019 16:46:03 +0000
Subject: [PATCH] Fixing an exception when the header already exists

---
 .../Enrichers/CorrelationIdHeaderEnricher.cs          | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/src/Serilog.Enrichers.CorrelationId/Enrichers/CorrelationIdHeaderEnricher.cs b/src/Serilog.Enrichers.CorrelationId/Enrichers/CorrelationIdHeaderEnricher.cs
index 66a1ea0..57b3517 100644
--- a/src/Serilog.Enrichers.CorrelationId/Enrichers/CorrelationIdHeaderEnricher.cs
+++ b/src/Serilog.Enrichers.CorrelationId/Enrichers/CorrelationIdHeaderEnricher.cs
@@ -52,8 +52,15 @@ private string GetCorrelationId()
             var correlationId = string.IsNullOrEmpty(header)
                                     ? Guid.NewGuid().ToString()
                                     : header;
-
-            _contextAccessor.HttpContext.Response.Headers.Add(_headerKey, correlationId);
+            
+#if NETFULL
+            if(!_contextAccessor.HttpContext.Response.Headers.AllKeys.Contains(_headerKey))
+#else
+            if (!_contextAccessor.HttpContext.Response.Headers.ContainsKey(_headerKey))
+#endif
+            {
+                _contextAccessor.HttpContext.Response.Headers.Add(_headerKey, correlationId);
+            }
 
             return correlationId;
         }