I'd like to collapse two strings `s1 = "word1 word2 word3"` and `s2 = "word2 word3 word4"` but removing the extra (future) consecutive overlap/duplicate (`"word2 word3"`). That is, I should obtain `s = "word1 word2 word3 word4`" rather than `s = "word1 word2 word3 word2 word3 word4"`. 

More simply, it should also work for single-word overlaps: `s1 = "word1 word2"` and `s2 = "word2 word3"` should give me `s = word1 word2 word3"` rather than `s = "word1 word2 word2 word3"`.

I am using `wordnumber` for illustration purposes but of course it should work for any word...