double

A double door, window, etc. includes two of the same kind.

Example: We opened the double door to let the piano come in.

A double accident, murder, etc. is two events that happen together or are somehow connected.

Example: With the double wedding, Sunday was the busiest day of the year.

A double thing is for two people.

Example: They slept together in a double bed, while James slept in a single in the other room.

Twice as big, twice as many, or twice as much as usual.

Example: I'll have a double coffee.

If something is double, it is 100% bigger than something else.

Example: The new computer was double the cost of the old one.