Long term capital gain tax on mutual fund. & ELSS

What is the impact on ltcg tax on mutual funds.?
Will there be double taxation?
I pay mutual funds to buy units
Mutual funds buy & sell stocks & pay tax
Then again i sell units and pay tax.

Secondly, what will become to ELSS Investments existing and future investments. ??
Will i have to pat ltcg for elss investments done 3 years ago when I redeem it now.

Are you sure about this?