in some areas a vote had already been cast by means of a cross or tick BEFORE the ballot paper was handed to the legitimate constituent to vote with?
I'd say it's very hard to believe. Voting fraud does exist in Poland, at least at a low level (a few years ago a Sołtys was found to have 42 people registered as living in his 3 bedroom house) but essentially things are done correctly in PL and something like that would have come up immediately, loudly and under media scrutiny.
Certain political tendencies like nothing more making than wild allegations, the wilder the better. Paradoxically, they are the ones most likely to get up to something dodgy.