In general, clients are paying any skilled laborer for their skills, not primarily their time. So I would consider it fine to charge the second client full price, ignoring all else. But if that goes against your conscience then don't do it.

Now with that being said in this situation you mention that they are in the same field. If them using the same business card could harm your original client in any way I would argue it's wrong to resell it as is, regardless of the cost. I would suggest in that case to say that to the second client and ask them to consider another similar design that you make.

In general, clients are paying any skilled laborer for their skills, not primarily their time. So I would consider it fine to charge the second client full price, ignoring all else. But if that goes against your conscience then don't do it.

Now with that being said in this situation you mention that they are in the same field. If them using the same business card could harm your original client in any way I would argue it's wrong to resell it as is, regardless of the cost. I would suggest in that case to say that to the second client and ask them to consider another similar design that you would make.