For TDC/Ductmate, gasket goes on one flange face only. Keep it relaxed and continuous through the corners; do not stretch it thin, double-gasket both ducts, or bolt over a gap/fold.
Ask when connector type, gasket, cleat/bolt schedule, pressure class, or manufacturer detail is unclear.
Watch out
Do not double-gasket both duct ends, stretch gasket to make it reach, or bolt over dirty, folded, missing, or displaced gasket.
Check
Confirm gasket is installed on one duct/flange face only.
Check corners first
most gaps, folds, and rolled gasket start there.
Keep the tape relaxed, not stretched thin.
Clean and dry the flange face before gasket.
If the joint is already tight over bad gasket, reopen it instead of burying it in mastic.
Steps
If the joint is not fully bolted, loosen as needed, re-seat or replace the gasket, and check all sides and corners before tightening.
If the joint is already tight, do not mastic over the gap. Reopen the joint, inspect the gasket, and replace it if it is torn, folded, or unevenly compressed.
Make sure the flange face is clean and dry before re-seating the gasket.
Check the corners closely because gasket gaps hide there first.
Do not call the joint sealed until the gasket is continuous all the way around.
Say this to your foreman
The gasket on this TDC/Ductmate joint [is missing / has a gap at ]. The joint [has not been bolted yet / is already bolted]. Do you want me to reopen it and fix the gasket now?