How to make a collect items to get badge script?

I really have no clue on how to script something like this.

I'd like the player to collect 6 items in-game (by the name of item1, item2, item3, etc..) and once the user has collected all 6 of them, he/she will be rewarded a badge. It is important that items cannot be double collected.

Could someone help me out with this?

Thanks in advance.