Fix duplicate newlines in object expression.

src/Fantomas.Tests/RecordTests.fs

@@ -597,4 +597,49 @@ type Element =
 
       /// The qualified name.
       Name: Name }
+"""
+
+[<Test>]
+let ``don't duplicate newlines in object expression, 601`` () =
+    formatSourceString false """namespace Blah
+
+open System
+
+module Test =
+    type ISomething =
+        inherit IDisposable
+        abstract DoTheThing: string -> unit
+
+    let test (something: IDisposable) (somethingElse: IDisposable) =
+        { new ISomething with
+
+            member __.DoTheThing whatever =
+                printfn "%s" whatever
+                printfn "%s" whatever
+
+            member __.Dispose() =
+                something.Dispose()
+                somethingElse.Dispose() }
+"""  config
+    |> prepend newline
+    |> should equal """
+namespace Blah
+
+open System
+
+module Test =
+    type ISomething =
+        inherit IDisposable
+        abstract DoTheThing: string -> unit
+
+    let test (something: IDisposable) (somethingElse: IDisposable) =
+        { new ISomething with
+
+            member __.DoTheThing whatever =
+                printfn "%s" whatever
+                printfn "%s" whatever
+
+            member __.Dispose() =
+                something.Dispose()
+                somethingElse.Dispose() }
 """

src/Fantomas/CodePrinter.fs

@@ -589,9 +589,25 @@ and genMemberBindingList astContext node =
     | [x] -> genMemberBinding astContext x
 
     | MultilineBindingL(xs, ys) ->
-        let prefix = sepNln +> col (rep 2 sepNln) xs (function 
-                                   | Pair(x1, x2) -> genPropertyWithGetSet astContext (x1, x2) 
-                                   | Single x -> genMemberBinding astContext x)
+        let prefix =
+            let genX =
+               function
+               | Pair(x1, x2) -> genPropertyWithGetSet astContext (x1, x2)
+               | Single x -> genMemberBinding astContext x
+
+            let sepNlnX (x': Composite<SynBinding, SynBinding>) =
+               match x' with
+               | Pair(x, _)
+               | Single x -> sepNln +> sepNlnConsideringTriviaContentBefore x.RangeOfBindingSansRhs
+
+            let firstSepNln =
+                match xs with
+                | (Pair(x,_))::_
+                | (Single x)::_ -> sepNlnConsideringTriviaContentBefore x.RangeOfBindingSansRhs
+                | _ -> sepNln
+
+            firstSepNln +> colEx sepNlnX xs genX
+
         match ys with
         | [] -> prefix
         | _ -> prefix +> rep 2 sepNln +> genMemberBindingList astContext ys