Exception: "An item with the same key has already been added" when comparing commits

I get ArgumentException when comparing two commits from https://github.com/shageman/the_next_big_thing with v.0.23.1.0

 System.ArgumentException:`  An item with the same key has already been added.
   at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
   at LibGit2Sharp.Patch.AddFileChange(git_diff_delta* delta) in c:\Git\LibGit2Sharp-2\LibGit2Sharp\Patch.cs:line 54
   at LibGit2Sharp.Patch..ctor(DiffHandle diff) in c:\Git\LibGit2Sharp-2\LibGit2Sharp\Patch.cs:line 43
   at LibGit2Sharp.Diff.<.cctor>b__1b(DiffHandle diff) in c:\Git\LibGit2Sharp-2\LibGit2Sharp\Diff.cs:line 104
   at LibGit2Sharp.Diff.BuildDiffResult[T](DiffHandle diff) in c:\Git\LibGit2Sharp-2\LibGit2Sharp\Diff.cs:line 120
   at LibGit2Sharp.Diff.Compare[T](Tree oldTree, Tree newTree, IEnumerable`1 paths, ExplicitPathsOptions explicitPathsOptions, CompareOptions compareOptions) in c:\Git\LibGit2Sharp-2\LibGit2Sharp\Diff.cs:line 253
   at LibGit2Sharp.Diff.Compare[T](Tree oldTree, Tree newTree) in c:\Git\LibGit2Sharp-2\LibGit2Sharp\Diff.cs:line 157

The code I'am using:
this.repository.Diff.Compare<Patch>(commitB.Tree, commitA.Tree);

commitA: a9a82df96ab1f73c69b2668b91eda31e672bbf98 Move rails application back to root to make heroku deployment work again
commitB: ede0b0c359d5af516889563bebafd51b7f43115a Gemnasium doesn't recognize Gemfile when its not in the root. Let's try symlinks

Start with LibGit2Sharp/Patch.cs at AddFileChange and review the call path from Diff.cs during Diff.Compare. Reproduce the comparison using commits a9a82df96ab1f73c69b2668b91eda31e672bbf98 and ede0b0c359d5af516889563bebafd51b7f43115a. Done means this comparison no longer throws the duplicate-key ArgumentException and returns the expected patch changes.