I guess there is nothing wrong if somebody tries to merge into your lane as long as he will not impede you, assuming he switched on his signals and has given enough time for you to react accordingly.

If you're speed is not enough to cover the gap between you and the car in front of you and the car trying to merge into the gap will not be endangering you and the car in front, I guess the best way is to just courteously let him merge.

Now, if the gap is too small and the speeds all of you are doing will not adequately allow him to merge, then he is either being a reckless a** or being stupid enough to not know what he's doing. However, we err on the side of caution so drive safely.