In music notation, a duplet is indicated by a number '2' drawn above or below two notes joined by a beam, usually with a bracket spanning the notes. It indicates that the notes indicated are to be evenly subdivided into halves of a beat rather than thirds.
It is similar in this way to a triplet, but is used when the time signature is a multiple of three -- for instance, 3/2 or 3/4 time. If two connected eighth notes are labeled as a duplet, then each one receives one-and-one-half beats instead of one beat.
A series of duplets on the staff looks something like this (complete with time signature):
/\
---| /------------------------|------------ 2 ---|----
|/ * | |\ |
---/-------------*-|-------*--|--|-|\-----|\-----|----
/| 3 *| | | | | | | | \ |
-/-|/\---------|-|-|----*-|---|--|-|-|----|--|---|----
| | | 2 | | | | | | * | | | | |
|--|--|--------|-|/----|--|---|---*--|---*---|---|----
\ | | |/ | / | * | |
--\|_/-----------------|/-----|-------------*----|----
| 2
\|