this is a good example of why context is important. Whether or not you can use 'do' in this sentence depends on what comes before it. If you were talking and the other person said - 'no problem, the neighbours won't see me', then you would use the 'do' in your sentence. Your listener thinks the neighbours won't see him, but you are saying, well if they DO see you..... However, as a stand alone sentence with no precursor then the 'do' would not be appropriate - it would sound odd. In that case, it is simply a 'if x, then y construction'.

It can be redundant. You can use "do" + verb to add emphasis. In this case, you're emphasizing that if the neighbors really see you, then, there will be resulting consequences. Perhaps using "do" in this case a stylistic choice rather than a necessary one.

no, you're sentence is OK. we sometimes use do/does/did for emphasising ! so ,that's ok to say!

The "do" adds emphasis. You can Google for "emphatic do" to see some examples and grammar. The "do" is not necessary, but it is not redundant either. It changes the mood or feeling of the sentence, but not the principal meaning.

Actually it's not a redundancy. You can use the auxiliary " do " to emphasize your idea. If you prefer drop it, almost no change of meaning.