So the Human Variant in 5E D&D gets two different ability points, proficiency in one skill and a feat.
Let's say that human is a cleric with acolyte background. He takes a double dose of religion (Am I wrong in presuming you can do this?), then the proficiency in religion and then the "Skilled Feat" with one of the three skill as religion, can they stack and how?
Best Answer
No, you cannot stack proficiencies.
You're either proficient with a skill/item/weapon and get to apply your proficiency bonus, or you're not; you can't be doubly or triply proficient in something. Some class features and special abilities may allow you to double your proficiency bonus in some situations, but these are always explicitly spelled out. As the PHB states on page 12, under the "Proficiency Bonus" heading:
And this is reiterated in further detail in the "Proficiency Bonus" section on page 173:
If you would be granted the same tool/skill proficiency by your background and your class during character generation, you get to just choose a different proficiency of the same type.
Extra: However, as mattdm's answer points out, Xanathar's Guide to Everything introduces a new feat, Prodigy, which amongst other things allows you to choose one skill you have proficiency in and double your proficiency bonus when using that skill - so as a variant human taking a feat at 1st level, you could improve your bonus to one skill in this way.