Typically you get the double points on the second and subsequent stays.
So a Plat using the Marriott Visa would get 30 pts per $ instead of 20.
I am underwhelmed.
Under a stay twice, get a free night promo, if I stay twice at a $100 a night FFI I get 4400 pts and a free Cat 4 or Cat 5 cert worth 20K or 25K points.
Under a double points promo, even after the qualifying stay, I get 6400 points for the two stays.
Hmm, 24,400 to 29,400 points or 6400 points, which is better?
Math:
Single points and cert:
Base 1000
Pt bonus 500
Pt gift 200
Visa 500
---------
2200 per night + half a Cat 4 cert = total reward 12,200 per night
Double points assuming qualifying stay already done:
Base 2000
Pt bonus 500
Pt gift 200
Visa 500
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3200 per night total reward = a tiny fraction of what we would get with the cert based promo