Do luxury booster from the same resource stack?

When I have enough units of a luxury resource to activate the booster twice, the game allows me to do that. Do I get double the effect of the booster or will one activation go to waste?

You won't get double the effect, but the duration is extended. E.g. if you use a boost and the effect lasts 20 turns, then you immediately use it again, it lasts 40 turns. So, the activation will not go to waste.

This is especially useful, if you have any temporary modifiers to increase the duration of a boost. E.g. in one game I had an effect ( either through tech or a great building), which increased the duration of boosters, depending on how many boosters I have active. This allowed me to get up to 36 turns of boost per activation. So, i could quickly stack up the booster duration to triple digit turns with a low resource cost.