Yes, each trade port on a planet gives the same amount of credits as the first one. They don't help your tradeport chain though, but if you don't have anything else to use the logistical slots for, they are a good investment.

Two trade ports = twice as much trade income.  This is the strength of trade ports, so long as you have logistics space you can add more income.

However, the longer your trade route, the more money an individual trade port will produce.  This means the best way to build your trade ports early on is one one each planet in a straight line to maximize that chain.  I personally don't even build trade ports unless I can get at least a chain of 4.