And was before was under Prussian control from the beginning of state formation in this area is not it? 
>This intolerance and disregard for any other citizenship or affiliation is shown over and over at wikipedia by many Polish people.
There is mentioned in the article: “Prussia (later Germany) thus he was a German citizen.”  I think you do not tolerate the thing the nationality is before that.
>The Polish law on Citizenship seems to supports something like this attitude. While dual citizenship is tolerated, no other citizenship than Polish is recognized.
You mean on territory of Poland. Yes I believe it is if you are Polish citizen and you enter Polish territory you can not disregard the any the duties as such. You can not be both at the same time on territory of Poland and claim different treatment and rights. Simple you can not switch with rights and duets as it is convenient at a moment. I think this is reasonable solution
>Poland only recognizes a dual citizenship person as being only Polish.
It is not so simple but something like I explained above.
>Johann Dzierzon was not born in Poland. His father, grandfather, etc were not born in Poland. They were all born in Lowkowitz, Silesia, Prussia, Germany.
You know, my perception is that in history Germans changed their minds. When was convenient the people were Germans citizens (subjects) on other time when convenient there were some individuals were set as alien element because their origin. This imperial politic injustice.
>The person Johann Dzierzon on Wikipedia is now Jan Dzierżon, completely overloaded with Polish-language sources. 
And it should be as Jan Dzierżon is Polish name and he recognized himself as Polish. Also his closest relatives in at least two next generation continued this way. 
>Many of those are already in the bottom references, so now we have the same Polish 'sources' plastered onto Dzierzon double or triple times.
Sorry. You can dislike it but “Further reading” is chosen set of most informative publications. That is other purpose. 
>Seems like complete overkill and against Wikipedia policy.
I do not know what you mean.
>No wonder Encyclopedia Britannica reflects this kind of overload .
There is also on some newest French encyclopaedias on line. I am sorry you are disappointed. It sometimes hapend that our own point of view is not the common one.
Best regards,