I have got this exception several times when working with the new spatial WKT support:
System.ArgumentException : An item with the same key has already been added.
  at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
  at NetTopologySuite.NtsGeometryServices.CreateGeometryFactory(IPrecisionModel precisionModel, Int32 srid, ICoordinateSequenceFactory coordinateSequenceFactory)
  at NetTopologySuite.IO.WKTReader.ReadGeometryTaggedText(IEnumerator`1 tokens)
  at NetTopologySuite.IO.WKTReader.Read(TextReader reader)
  at NetTopologySuite.IO.WKTReader.Read(String wellKnownText)
  at Spatial4n.Core.Io.NtsShapeReadWriter.ReadShape(String str) in c:\Work\Spatial4n\Spatial4n.Core\Io\NtsShapeReadWriter.cs: line 99
  at Raven.Database.Indexing.SpatialIndex.MakeQuery(SpatialStrategy spatialStrategy, String shapeWKT, SpatialRelation relation, Double distanceErrorPct) in c:\Builds\RavenDB-Unstable-v1.2\Raven.Database\Indexing\SpatialIndex.cs: line 50
  at Raven.Database.Indexing.Index.IndexQueryOperation.GetLuceneQuery() in c:\Builds\RavenDB-Unstable-v1.2\Raven.Database\Indexing\Index.cs: line 983
  at Raven.Database.Indexing.Index.IndexQueryOperation.<Query>d__41.MoveNext() in c:\Builds\RavenDB-Unstable-v1.2\Raven.Database\Indexing\Index.cs: line 753
  at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
  at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
  at System.Collections.Generic.List`1.InsertRange(Int32 index, IEnumerable`1 collection)
  at Raven.Database.DocumentDatabase.<>c__DisplayClass88.<Query>b__7e(IStorageActionsAccessor actions) in c:\Builds\RavenDB-Unstable-v1.2\Raven.Database\DocumentDatabase.cs: line 950
  at Raven.Storage.Managed.TransactionalStorage.ExecuteBatch(Action`1 action) in c:\Builds\RavenDB-Unstable-v1.2\Raven.Database\Storage\Managed\TransactionalStorage.cs: line 132
  at Raven.Storage.Managed.TransactionalStorage.Batch(Action`1 action) in c:\Builds\RavenDB-Unstable-v1.2\Raven.Database\Storage\Managed\TransactionalStorage.cs: line 112
  at Raven.Database.DocumentDatabase.Query(String index, IndexQuery query) in c:\Builds\RavenDB-Unstable-v1.2\Raven.Database\DocumentDatabase.cs: line 955
  at Raven.Client.Embedded.EmbeddedDatabaseCommands.Query(String index, IndexQuery query, String[] includes, Boolean metadataOnly) in c:\Builds\RavenDB-Unstable-v1.2\Raven.Client.Embedded\EmbeddedDatabaseCommands.cs: line 373
  at Raven.Client.Document.AbstractDocumentQuery`2.ExecuteActualQuery() in c:\Builds\RavenDB-Unstable-v1.2\Raven.Client.Lightweight\Document\AbstractDocumentQuery.cs: line 504
  at Raven.Client.Document.AbstractDocumentQuery`2.GetEnumerator() in c:\Builds\RavenDB-Unstable-v1.2\Raven.Client.Lightweight\Document\AbstractDocumentQuery.cs: line 662
  at System.Linq.Enumerable.Any(IEnumerable`1 source)
  at Raven.Client.Linq.RavenQueryProviderProcessor`1.GetQueryResult(IDocumentQuery`1 finalQuery) in c:\Builds\RavenDB-Unstable-v1.2\Raven.Client.Lightweight\Linq\RavenQueryProviderProcessor.cs: line 1288
  at Raven.Client.Linq.RavenQueryProviderProcessor`1.ExecuteQuery() in c:\Builds\RavenDB-Unstable-v1.2\Raven.Client.Lightweight\Linq\RavenQueryProviderProcessor.cs: line 1213
  at Raven.Client.Linq.RavenQueryProvider`1.System.Linq.IQueryProvider.Execute(Expression expression) in c:\Builds\RavenDB-Unstable-v1.2\Raven.Client.Lightweight\Linq\RavenQueryProvider.cs: line 183
  at System.Linq.Queryable.Any(IQueryable`1 source)
  at Geo.Tests.Raven.RavenTests.IndexPropertyTest(IGeometry geometry) in RavenTests.cs: line 69