Sometimes a father is sued for child support - or threatened to be sued - demanding support despite already supporting a child from a previous relationship. There's a couple of options here to ensure proper credit.
Prior Support paid pursuant to order/agreement
If the support you are paying is pursuant to a prior order or prior valid, written agreement (usually a "stipulation of settlement" or "separation agreement"), then you can bring that prior order or agreement with you to court. So long as you also bring proof you are actually paying support pursuant to that prior order or agreement, you should get a credit. The credit works by subtracting the prior support amount off one's adjusted gross income before calculating support for the subsequent child.
If you don't have a prior order/agreement
If you do not have a prior order or valid, written agreement, then try to get one in place before the mother of the subsequent child files for child support. If you do an agreement, make sure to have it drafted by an experienced Child Support lawyer, as not "any old agrement" will do. If the subsequent mother has already filed, then the only way you can get credit is to claim (if it's the case) that after subtracting the support order for the subsequent child, the prior child will not be left with enough money to minimally support that child.