Disadvantage

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

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

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

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