Per National Mah Jongg League rules, if two or more players call for the same tile:
The player closest to the discarding player (in counter-clockwise order) takes precedence
If a player is calling for Mah Jongg, they take precedence over players calling for an exposure
In our online game, the computer will wait until everyone has made their decision before allocating the discard (as it needs to check who's closest to the discarder and whether a player is calling for MJ).
For more information on this rule (and other rules), check out this tutorial.