Workflow issues

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Hello,

I am hoping someone can provide some advice for debugging Workflow.
An issue we have encountered is that extra items appear in the 'Waiting For Me' page; We also occasionally get a 'Duplicate Entry' errors when items are submitted to Workflow.
Updating to the latest version, (5.7.13) from 5.7.5.6 does not seem to have removed the issue where 'approving' workflow submissions then removes other workflow items.
Could this be related to the PagePermission tables being corrupted in some way?
Thanks in advance for any help.