nuances - Meaning of last "te" of saki ni itte te (先に行ってて)

I always hear people saying

　　　先に行ってて　which means 'go ahead' 

But I don't really understand the meaning of the last て. I know it's the abbreviation of いて but if I translate it to English, I always think like this.

　　　先に行っていて - Going ahead

So anyone can explain the latter 'te' of this sentence?

Answer

As you said, 「行ってて」 is the colloquial contraction of 「行っていて」 and we almost always use the contracted form in informal speech.

There is, however, a fairly big difference in meaning between 「先に行って」 and 「先に行ってて」.

「先に行って」 simply means "You go first."

「先に行ってて」 means "You go first and wait for me/us." or "You go first and I/We will follow you."