When using Alolan Raichu's "Electro Rain" attack or Hoopa GX's "Devilish Hands GX" attack with a Telescopic Sight tool attached, would Telescopic Sight boost the damage to Pokemon-GX and Pokemon-V each and every time I choose that Pokemon?

Per the rule book you make all your choices first, calculate the damage, then apply modifiers (such as Telescopic Sight), and then apply damage to the chosen Pokemon. So you would not get to add +30 more than once if you choose the same Pokemon again and again; however if you choose multiple Pokemon-GX or Pokemon-V, you would get to add +30 to each of those.

Relevant cards

SM9_96

SM11_57

SWSH4_160