diff --git a/src/Compiler/Interactive/fsi.fs b/src/Compiler/Interactive/fsi.fs
index 7c634ca81d6..c31f210022f 100644
--- a/src/Compiler/Interactive/fsi.fs
+++ b/src/Compiler/Interactive/fsi.fs
@@ -1797,6 +1797,9 @@ type internal FsiDynamicCompiler
 
         let multiAssemblyName = ilxMainModule.ManifestOfAssembly.Name
 
+        // The name of the assembly is "FSI-ASSEMBLY" for all submissions. This number is used for the Version
+        let dynamicAssemblyId = Interlocked.Increment &dynamicAssemblyId
+
         // Adjust the assembly name of this fragment, and add InternalsVisibleTo attributes to
         // allow internals access by all future assemblies with the same name (and only differing in version)
         let manifest =
@@ -1811,13 +1814,10 @@ type internal FsiDynamicCompiler
             { manifest with
                 Name = multiAssemblyName
                 // Because the coreclr loader will not load a higher assembly make versions go downwards
-                Version = Some(parseILVersion $"0.0.0.{maxVersion - dynamicAssemblyId}")
+                Version = Some(parseILVersion $"0.0.0.{maxVersion - dynamicAssemblyId % maxVersion}")
                 CustomAttrsStored = storeILCustomAttrs (mkILCustomAttrs attrs)
             }
 
-        // The name of the assembly is "FSI-ASSEMBLY" for all submissions. This number is used for the Version
-        dynamicAssemblyId <- (dynamicAssemblyId + 1) % maxVersion
-
         let ilxMainModule =
             { ilxMainModule with
                 Manifest = Some manifest
diff --git a/src/Compiler/Interactive/fsihelp.fs b/src/Compiler/Interactive/fsihelp.fs
index eb1efd22ed7..f21da3ab441 100644
--- a/src/Compiler/Interactive/fsihelp.fs
+++ b/src/Compiler/Interactive/fsihelp.fs
@@ -14,6 +14,7 @@ open FSharp.Compiler.IO
 module Parser =
 
     open System.Xml
+    open System.Collections.Concurrent
 
     type Help =
         {
@@ -79,22 +80,19 @@ module Parser =
         let s = if idx > 0 then s.Substring(0, idx) else s
         s
 
-    let xmlDocCache = Dictionary<string, string>()
+    let xmlDocCache = ConcurrentDictionary<string, Lazy<XmlDocument>>()
 
     let tryGetXmlDocument xmlPath =
-        try
-            match xmlDocCache.TryGetValue(xmlPath) with
-            | true, value ->
-                let xmlDocument = XmlDocument()
-                xmlDocument.LoadXml(value)
-                Some xmlDocument
-            | _ ->
+        let valueFactory xmlPath =
+            lazy
                 use stream = FileSystem.OpenFileForReadShim(xmlPath)
                 let rawXml = stream.ReadAllText()
                 let xmlDocument = XmlDocument()
                 xmlDocument.LoadXml(rawXml)
-                xmlDocCache.Add(xmlPath, rawXml)
-                Some xmlDocument
+                xmlDocument
+
+        try
+            Some(xmlDocCache.GetOrAdd(xmlPath, valueFactory).Value)
         with _ ->
             None