No, in the UK you don't tip on top if there is a service charge on the bill. Here it seems to be expected.
I sometimes give something on top. Not always. Depends if I think the service was beyond what it should have been and if I have any cash on me. I have asked my Dominican friends and they are all saying they do the same and they say the Propina Legal is the Propina.
I frequently go back to the same places and have not noticed a difference in service if I tipped or not the last time.
If I think someone is giving wonderful service I do always give a good review on Google and name them. This helps the business and the person and is a nice thing to do.