They keys that make a release or iteration unique are the same name and start & end dates. If these are all the same, they show as one listing. Conversely, if anything is different, you may see duplicates listed.
Here's an example of a release but the same holds true for an iteration.
This screenshot shows a duplicate listing of release "2017 Q2".
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Reviewing this release under Timeboxes, we see that the Release Date for one of the projects is out of sync with the others.
Navigate to the top level project with project scoping to include child projects and make sure that all Start Dates and Release Dates are identical for each project listed with this release.
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Correcting the date, they are now all in sync.
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Now when we review the listing, there is only one entry for 2017 Q2.
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To query for synced Releases using the API - query for Name, ReleaseStartDate, and ReleaseDate. Validate that all three of these values are identical. *Note that the SyncedWithParent attribute cannot be used in queries. Also, the SyncedWithParent field is set only if the Release was originally created with the "Cascade Projects" option. It does not change if the Release becomes out of sync with its Parent.