Other question.. what is the difference between setelah and sesudah?
I think setelah (telah) and sesudah (sudah) have the same meaning. However, telah and sudah have different and similar uses.
In English the second enter is redundant methinks
You are right. It is rarely used.
"I will enter after you entered" was wrong. It sounds better to me though
That's not quite right. "I will enter after you have entered" is correct. Or can contract to "you've".
Could also accept "I will enter once you have entered" which has the same meaning, of "after" or "as soon as".
It is the same meaning as aku/saya akan masuk setelah kamu masuk.