Non-Custom Workflow error: An item with the same key has already been added.

We have 6 workflows all triggered off of the same field change on an entity.  These workflow are simple and have an IF statement and then create a related record.

Occasionally (1 out of 50 times or so) one will fail with the error "An item with the same key has already been added."

I was told by support: "We know this type of workflow conflict will intermittently cause errors."   ....after 3 months of investigation.

I thought the workflows processed Async and sequentially, so that type of error seems impossible in normal processing.  I have also never seen this error before (and neither have they) and it started after a failed upgrade and I feel that is not coincidental.  (We are on MSCRM Online).

I know I can combine workflows and/or use child workflows, but I feel there is some other issue going on and I have never read anywhere that you can have this type of conflict.

Does this make sense to anyone else?  I'd like to get a more definitive answer before we re-do all of our workflows and hope we don't get the same error.

Thank you,

Dan