Our Diwali sale brought back loads of old users. Do they count as new installs?

In most default setups they sit in the same install count. Someone who installed before, lapsed beyond the earlier attribution window and then tapped a festive ad is a reattribution: the new campaign legitimately gets credit, but the user is re-acquired, not acquired. Counting both together lowers reported CPI without any real efficiency gain, and mixed cohorts distort LTV models, because returning users often monetise faster. Look for the reattribution flag at install level in your attribution platform and split the festive cohort into acquisition and reactivation. Judge new users on cost per activated new user and 30-day retention, and returning users on cost per returning active user and repeat transaction rate. There is no reliable published benchmark for reattribution rates, so measure your own baseline outside the festive window and compare. Next season, split the campaigns before the window opens.