If Twinblade Paladin gains double strike after combat damage has already been dealt (most likely because a creature with lifelink dealt combat damage), Twinblade Paladin will deal only regular combat damage.

Twinblade Paladin's first ability triggers just once for each life-gaining event, whether it's 1 life from Dazzling Angel or 4 life from Apothecary Stomper.

Each creature with lifelink dealing combat damage causes a separate life-gaining event. For example, if two creatures you control with lifelink deal combat damage at the same time, Twinblade Paladin's first ability will trigger twice. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, planeswalkers, and/or battles at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once.

In a Two-Headed Giant game, life gained by your teammate won't cause Twinblade Paladin's first ability to trigger, even though it caused your team's life total to increase.

In a Two-Headed Giant game, Twinblade Paladin has double strike as long as your team has 25 or more life.

If you gain an amount of life "for each" of something or "equal to the number" of something, that life is gained as one event and Twinblade Paladin's first ability triggers only once.

In a Two-Headed Giant game, life gained by your teammate won't cause the first ability to trigger, even though it caused your team's life total to increase.

If Twinblade Paladin is dealt lethal damage at the same time that you gain life, it won't receive a counter from its first ability in time to save it.

If you're dealt first-strike combat damage that brings your life total below 25, Twinblade Paladin will immediately lose double strike. It won't deal regular combat damage unless you bring your life total back above 25 before the regular combat damage step.