Dear Desdemona,
There is very little about Joseph in the Bible. On the basis of the evidence in the Bible, one could say that Joseph was not previously married, or that he was. The Bible is silent on this point.
However, the Protoevangelium of James does record that Joseph was a widower with children. Here is a URL to an Internet copy of this document:
http://www.newadvent.org/fathers/0847.htm
Of course, this document is not Sacred Scripture, but it is a very early document, from about AD 120 or so. So, St. Jerome was defending a very ancient teaching when he wrote against the heretic Helvidius.
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