No. You get to add sneak attack damage for every successful grapple check you use to deal damage or pin though. Constrict allows you to deal extra damage on every successful grapple check. They have the same rules for activation (basically, strangle is slightly more restricted because you don't deal the damage if you move the enemy).
So, on a single successful grapple check (which you deal damage or pin) you can do constrict damage and strangle (sneak attack) damage (and whatever kind of weapon you're using). Not just one or the other. However, you can get rapid grappler and greater grapple.
With rapid grappler and greater grapple you can make up to 3 grapple checks a round (one as a standard, one as a move, and one as a swift but requires you to start the round already grappling someone to do so). If on all 3 checks you decided to deal damage instead of move or pin the creature then each successful check would allow you to deal both constrict and strangle damage.