This issue/problem was introduced with the possibility to assign DOIs to galleys and the possibility to have the public identifiers as DOI suffix. -- Because the DOIs have to be unique, the public galley identifiers had to be unique within the journal (and not within an article any more).
We decided to remove this suffix option for DOIs and the uniqueness check for public galley identifiers within a journal i.e. there should only be the uniqueness check for public galley identifiers within an article. That would solve you problem.
I've started working on this, but it will maybe take some time, because I will also have to make changes in DOI and URN plug-in as well as to consider the upgrade (i.e. the users that are using public galley identifiers as DOI suffix).
If you don't use DOIs (or URNs) in that way, you are not affected and don't have to wait for the whole solution i.e. you could already apply these changes: https://github.com/bozana/ojs/commit/86 ... 3d0a307f99
Be aware that the commit/changes are for the current ojs-dev-2_4 branch i.e. that the code (in classes/submission/form/ArticleGalleyForm.inc.php) could be a little bit different than yours.
I hope that this will help!