undo what wasn't necesary

Author: nealmummau

src/SqlServer/src/Eventuous.SqlServer/Scripts/2_AppendEvents.sql

@@ -29,8 +29,9 @@ BEGIN
         @current_version  = @current_version OUTPUT,
         @stream_id        = @stream_id OUTPUT;
 
+    SET @new_version = @current_version + @count_messages;
+
     BEGIN TRY
-        SET @new_version = @current_version + @count_messages;
 
         /*
             If another writer raced us, the unique constraint (StreamId,StreamPosition) will throw here.
@@ -84,7 +85,8 @@ BEGIN
     UPDATE s
     SET [Version] = @new_version
     FROM __schema__.Streams s
-    WHERE s.StreamId = @stream_id;
+    WHERE s.StreamId = @stream_id
+    AND s.[Version] = @current_version;
 
     -- final GlobalPosition value to return
     SELECT @position = (