It stacks as long as you have techer as a class. As Bellion stated, a techer main only skill makes it take only 1 cast, instead of 3. If you would like to check for yourself, go as a Fo/Te, cast shifta or deband three times, and wait for each four ticks to hit, then cast the 2nd and 3rd set of shifta/deband. Then go to your menu via esc on keyboard, and check Character Info. It will have the current time for any buffs you have at the time you clicked it. It will also show exp boost, rdr, and tri boosts, etc.