Please see below answer...

AFAIK you can have as many UINQUEs on a table as you want. But anyway:
You
can make sure by using a before InsertUpdate-trigger checking for an
exists(select FIELD from table where FIELD=VALUE) on your FIELD and
VALUE.
..............................

....My question..
How to define UNIQUE on a column>?
Also..what is VALUE above, a variable declared in trigger..?
thks