I had similar bug. This 'cause Joom!Fish 2.5 works another way then old versions. It keeps both articles (original and translated) in article manager, so articles anyway can't be stored with same aliases.
What I did: go to /libraries/joomfish/contentelement/contentelements/
You can see *.xml files stored here. Every of this files is corresponding for different elements. For example open content.xml and find this code:
<treatment>
<target>native</target>
<tableclass>Content</tableclass>
<publishedfield>state</publishedfield>
<translationObjectModel file="joomfish.translatable.translationobject.content">TranslationObjectContent</translationObjectModel>
<jformModel>ContentJForm</jformModel>
</treatment>
and remove this.
After you do this Joom!Fish stops duplicate contents, and you'll get many translations with same alias!
P.S.: You may need to remove duplicate articles in Article Manager.