His (my bf's) mum found out when the IRS (the US version of the Inland Revenue) sent a letter to their house (in the US, if you're married, you're both responsible for the tax bill) auditing them b/c his dad was claiming dependents in two different states.
What a way to find out, eh?
Anyhow, he had 'married' the other wife in the other state, even though of course the marriage wasn't valid.
He was imprisoned for bigamy.