API UnitIsUnit

Returns true if the two specified units are the same, false otherwise. isSame = UnitIsUnit(unit1, unit2);

Arguments

 * unit1 : UnitId - The first unit to query (e.g. "party1", "pet", "player")
 * unit2 : UnitId - The second unit to compare it to (e.g. "target")

Returns

 * isSame : Boolean - 1 if the two units are the same entity, nil otherwise.

Example
if ( UnitIsUnit("targettarget", "player") ) then message("Look at me, I have aggro from " .. UnitName("target") .. "!"); end;

Result
Displays a message if your target is targetting you.