Tautological adspeak and poor at that. You can't be 'first and only'. When you were first, you were the only; if you are the only, then you were, and are, the first.
You might consider 'the first and still the only'. Don't: 'only' is sufficient here. However, 'still the only' works as a shorter, punchier, phrase and makes a point about your uniqueness and the lack of serious competition. And it's the same number of words as the first, incorrect effort.
There, that was easy, wasn't it?