Update STRDIAG006.md - Duplicated line removed

Author: VaclavElias

en/diagnostics/STRDIAG006.md

@@ -17,7 +17,6 @@ public class STRDIAG006
 {
     // non existent setters count as non visible
     [DataMember(DataMemberMode.Assign)]
-    [DataMember(DataMemberMode.Assign)]
     public int Property1 { get; }
 
     [DataMember(DataMemberMode.Assign)]
@@ -57,4 +56,4 @@ To resolve the warning, increase the accessibility of the properties set to `pub
 
 ## References
 
-- [Serialisation](../manual/scripts/serialization.md)
+- [Serialisation](../manual/scripts/serialization.md)