diff --git a/src/fsharp/CompileOps.fs b/src/fsharp/CompileOps.fs
index 0b0414ac5a1..0cab042759a 100644
--- a/src/fsharp/CompileOps.fs
+++ b/src/fsharp/CompileOps.fs
@@ -4,11 +4,12 @@
 module internal Microsoft.FSharp.Compiler.CompileOps
 
 open System
+open System.Collections.Concurrent
+open System.Collections.Generic
 open System.Diagnostics
-open System.Text
 open System.IO
-open System.Collections.Generic
 open System.Runtime.CompilerServices
+open System.Text
 
 open Internal.Utilities
 open Internal.Utilities.Text
@@ -3509,14 +3510,14 @@ let PostParseModuleSpecs (defaultNamespace, filename, isLastCompiland, ParsedSig
     ParsedInput.SigFile(ParsedSigFileInput(filename, qualName, scopedPragmas, hashDirectives, specs))
 
 /// Checks if a module name is already given and deduplicates the name if needed.
-let DeduplicateModuleName (moduleNamesDict:Dictionary<string, Set<string>>) (paths: Set<string>) path (qualifiedNameOfFile: QualifiedNameOfFile) =
+let DeduplicateModuleName (moduleNamesDict:IDictionary<string, Set<string>>) (paths: Set<string>) path (qualifiedNameOfFile: QualifiedNameOfFile) =
     let count = if paths.Contains path then paths.Count else paths.Count + 1
     moduleNamesDict.[qualifiedNameOfFile.Text] <- Set.add path paths
     let id = qualifiedNameOfFile.Id
     if count = 1 then qualifiedNameOfFile else QualifiedNameOfFile(Ident(id.idText + "___" + count.ToString(), id.idRange))
 
 /// Checks if a ParsedInput is using a module name that was already given and deduplicates the name if needed.
-let DeduplicateParsedInputModuleName (moduleNamesDict:Dictionary<string, Set<string>>) input =
+let DeduplicateParsedInputModuleName (moduleNamesDict:IDictionary<string, Set<string>>) input =
     match input with
     | ParsedInput.ImplFile (ParsedImplFileInput.ParsedImplFileInput(fileName, isScript, qualifiedNameOfFile, scopedPragmas, hashDirectives, modules, (isLastCompiland, isExe))) ->
         let path = Path.GetDirectoryName fileName
@@ -3525,7 +3526,7 @@ let DeduplicateParsedInputModuleName (moduleNamesDict:Dictionary<string, Set<str
             let qualifiedNameOfFile = DeduplicateModuleName moduleNamesDict paths path qualifiedNameOfFile
             ParsedInput.ImplFile(ParsedImplFileInput.ParsedImplFileInput(fileName, isScript, qualifiedNameOfFile, scopedPragmas, hashDirectives, modules, (isLastCompiland, isExe)))
         | _ ->
-            moduleNamesDict.Add(qualifiedNameOfFile.Text, Set.singleton path)
+            moduleNamesDict.[qualifiedNameOfFile.Text] <- Set.singleton path
             input
     | ParsedInput.SigFile (ParsedSigFileInput.ParsedSigFileInput(fileName, qualifiedNameOfFile, scopedPragmas, hashDirectives, modules)) ->
         let path = Path.GetDirectoryName fileName
@@ -3534,7 +3535,7 @@ let DeduplicateParsedInputModuleName (moduleNamesDict:Dictionary<string, Set<str
             let qualifiedNameOfFile = DeduplicateModuleName moduleNamesDict paths path qualifiedNameOfFile
             ParsedInput.SigFile (ParsedSigFileInput.ParsedSigFileInput(fileName, qualifiedNameOfFile, scopedPragmas, hashDirectives, modules))
         | _ ->
-            moduleNamesDict.Add(qualifiedNameOfFile.Text, Set.singleton path)
+            moduleNamesDict.[qualifiedNameOfFile.Text] <- Set.singleton path
             input
 
 let ParseInput (lexer, errorLogger:ErrorLogger, lexbuf:UnicodeLexing.Lexbuf, defaultNamespace, filename, isLastCompiland) = 
diff --git a/src/fsharp/CompileOps.fsi b/src/fsharp/CompileOps.fsi
index 8f782ccee16..77916b63f49 100755
--- a/src/fsharp/CompileOps.fsi
+++ b/src/fsharp/CompileOps.fsi
@@ -65,10 +65,10 @@ val ComputeQualifiedNameOfFileFromUniquePath: range * string list -> Ast.Qualifi
 val PrependPathToInput: Ast.Ident list -> Ast.ParsedInput -> Ast.ParsedInput
 
 /// Checks if a module name is already given and deduplicates the name if needed.
-val DeduplicateModuleName: Dictionary<string,Set<string>> -> Set<string> -> string -> Ast.QualifiedNameOfFile -> Ast.QualifiedNameOfFile
+val DeduplicateModuleName: IDictionary<string,Set<string>> -> Set<string> -> string -> Ast.QualifiedNameOfFile -> Ast.QualifiedNameOfFile
 
 /// Checks if a ParsedInput is using a module name that was already given and deduplicates the name if needed.
-val DeduplicateParsedInputModuleName: Dictionary<string,Set<string>> -> Ast.ParsedInput -> Ast.ParsedInput
+val DeduplicateParsedInputModuleName: IDictionary<string,Set<string>> -> Ast.ParsedInput -> Ast.ParsedInput
 
 val ParseInput: (UnicodeLexing.Lexbuf -> Parser.token) * ErrorLogger * UnicodeLexing.Lexbuf * string option * string * isLastCompiland:(bool * bool) -> Ast.ParsedInput
 
diff --git a/src/fsharp/fsc.fs b/src/fsharp/fsc.fs
index 8aea32c097e..e18f782dc21 100644
--- a/src/fsharp/fsc.fs
+++ b/src/fsharp/fsc.fs
@@ -13,6 +13,7 @@
 module internal Microsoft.FSharp.Compiler.Driver 
 
 open System
+open System.Collections.Concurrent
 open System.Collections.Generic
 open System.Diagnostics
 open System.Globalization
@@ -1758,7 +1759,7 @@ let main0(ctok, argv, legacyReferenceResolver, bannerAlreadyPrinted, openBinarie
     
     let inputs =
         // Deduplicate module names
-        let moduleNamesDict = Dictionary<string,Set<string>>()
+        let moduleNamesDict = ConcurrentDictionary<string,Set<string>>()
         inputs
         |> List.map (fun (input,x) -> DeduplicateParsedInputModuleName moduleNamesDict input,x)
 
diff --git a/src/fsharp/vs/IncrementalBuild.fs b/src/fsharp/vs/IncrementalBuild.fs
index 5b429b2e708..6145eabe330 100755
--- a/src/fsharp/vs/IncrementalBuild.fs
+++ b/src/fsharp/vs/IncrementalBuild.fs
@@ -4,8 +4,9 @@ namespace Microsoft.FSharp.Compiler
 
 
 open System
-open System.IO
+open System.Collections.Concurrent
 open System.Collections.Generic
+open System.IO
 open System.Threading
 open Microsoft.FSharp.Compiler
 open Microsoft.FSharp.Compiler.NameResolution
@@ -1239,7 +1240,7 @@ type IncrementalBuilder(tcGlobals, frameworkTcImports, nonFrameworkAssemblyInput
         cache.GetFileTimeStamp filename
 
     // Deduplicate module names
-    let moduleNamesDict = Dictionary<string, Set<string>>()
+    let moduleNamesDict = ConcurrentDictionary<string, Set<string>>()
                             
     /// This is a build task function that gets placed into the build rules as the computation for a VectorMap
     ///