At now IEF module have own function for clone entity. But Drupal have popular Replicate module, that have extended support of cloning entities, with a lot of contrib modules. So using internal direct clone function can got problems with creating clones.
Very often problem is that IEF clone function don't create copies of entity reference targets. In most of sites this problem fixed via Replicate module and some hooks or additional modules like https://www.drupal.org/sandbox/jojonaloha/2374979 →
So when we clone entity via Replicate module - all works well, but when clone via IEF - got problems because exists hooks for Replicate module are ignored.
Will be good to integrate support for Replicate module into Inline Entity Form, for extend cloning feature with support of all contrib modules.
Closed: won't fix
1.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Due to limited developer time old feature requests for IEF are being closed. If you still have a need for this functionality, then you may feel free to reopen the issue. But please note that if you are still using Drupal 7 and would like for this functionality to be added to the D7-compatible version of IEF, then our limited resources and lack of interest may prevent that from ever happening.