When Ana shoots, the healing or damage effect is applied over a short interval. Assuming a second shot lands before this interval is up, will the target receive double the healing or damage? Or does the second shot override the first shot?
I believe that the target will receive the full amount of healing, that is to say, if one shot from her Biotic Rifle deals 70 damage/healing, two shots will deal 140 damage/healing. This can be experienced during the no limits matches where multiple Ana are healing one target, and the target is back to full health extremely quickly.
The overlap that occurs from a single Ana is not considerable during a match usually, as the only way it can occur is if you were to shoot a projectile shot followed by a hitscan shot (unscoped then scoped). This is due to the length of the heal over time being 0.6 seconds and the time between possible shots being 0.8 seconds. Scoping also takes 0.5 seconds, so mistiming the scope in after the first projectile shot decreases the likelihood of the overlap occurring.
Source for belief: 300+ hours on Ana in competitive
Source for exact numbers: https://overwatch.fandom.com/wiki/Ana