Advantage

For a comprehensive summary of the mechanics behind all rolls and modifiers, see: Die Rolls

When you roll with Advantage, you perform the roll twice, and use the higher result. You can only have Advantage once, in other words, you never roll more than twice due to Advantage.

Example: You roll two  d20 for an Attack Roll, because you have Advantage. The results are 16 and 4. Your effective result is 16.

Advantage is negated by Disadvantage. The number of Advantages and Disadvantages you have is irrelevant. Even if you have three sources of Advantage, a single source of Disadvantage will negate it.