A customer sent us a pdf that has this problem: when it is signed twice by pdfbox 1.8.x the second signature invalidates the first one.
If we apply the same procedure using pdfbox 2.0.x the problem doesn't occur, but the customer required java 1.5 so we can't switch to the new version in this case.
For we had anonymized the original PDF file by editing 3 stream inside the pdf, without altering the original structure. So the file "92752146_noSign_anonymous.pdf" you can find in attachement has not the original text/image streams, but reproduces the problem as the original one.
Thank you in advance