@Siedlerchr @cMadan I guess I can explain what happens and it looks like the mistake is coming from de.undercouch.citeproc.bibtex.BibTeXConverter#toItemData(org.jbibtex.BibTeXEntry). The rundown is as follows:

• User clicks on an entry which is then sent to create the preview.
• We create an org.jbibtex.BibTeXEntry instance from the information we have. Since we only have the journal name in one field, we create only one field inside this BibTeXEntry with key "journal` (or smth similar) and the name of the journal as value
• This created instance is sent to de.undercouch.citeproc.bibtex.BibTeXConverter#toItemData(org.jbibtex.BibTeXEntry) which creates the representation that is used in the CSL formatting engine

The last step creates the following representation for a simple article entry. Please note the double entry collection-title and container-title. When you look at the .csl file you can easily spot what happens. There is a macro containter with several conditional expressions. An "article" goes to the last else branch which states

<else>
    <group prefix=". " suffix="," delimiter=", ">
        <text variable="container-title" form="short" font-style="italic"/>
        <text variable="collection-title"/>
    </group>
</else>

My guess is that this is really meant to contain the Journal name and the Collection name, but I'm unable to create an example where this works. I tried with an example of InCollection but I always end up with a double "BookTitle".

Originally posted by @halirutan in #3259 (comment)