In PHB, it specifies that if you gain the same proficiency from two different sources, you may choose a different prof of the same type (skill or tool) instead.

Does the survivalist rule for the Scout Rogue in XGE contradict this by stating"-if you don't already have it"? pic.twitter.com/jF1kzoCOyc

— Big Red (@ThisTinSoldier) May 30, 2018

The proficiency rule for backgrounds applies when you gain your background at 1st level. It has no relevance outside that context.

— Jeremy Crawford (@JeremyECrawford) May 30, 2018

@JeremyECrawford Curse of Strahd: Champion Fighter accepts a dark gift, it lets him cast Contagion (it requires a melee spell attack). The Champion doesn't have any Spellcasting Ability (Charms, on DMG, don't cover this).
Which modifier must use a Champion to hit with Contagion?

— Draconis (@DerynDraconis) June 7, 2018

Sometimes a non-spellcaster gains the ability to cast a spell. If you're in that situation and a rule doesn't tell you how to calculate your spell's save DC, use the default calculation in the spellcasting rules (PH, 205), and your spellcasting ability modifier is 0. #DnD https://t.co/F4WYUluw7h

— Jeremy Crawford (@JeremyECrawford) June 11, 2018