Your skill bonus can not be added to a single die
roll or another number more than once. – D & D Principles V0.1 Chapter 1, p. 7
You can only receive one Competency Bonus once, regardless of how many resources you have. When selecting abilities and tool skills that have been awarded from a background or class, players should try to avoid overlapping abilities as the benefits are zero
However, a performance bonus may be increased or decreased depending on the circumstances
Ability bonus can be changed (doubled or halved,
for example) before you apply it. If a circumstance
suggests that your skill bonus is more than worth it
once on the same role or that it should be multiplied
more than once, but you only add it once
multiply it only once and halve it only once. – D & D Principles V0.1 Chapter 1, p. 7
DMs can use this to award a bonus for skill checks based on good role play and / or good planning.
The Rogue's Expertise feature is an exception
In the first level, choose two of your abilities, or
one of your skills and your knowledge with
Tools of thieves. Your proficiency bonus is doubled for everyone
Capability checking that you perform using one of the selected methods
On the 6th level you can choose two more
Skills. – D & D Principles V0.1 Chapter 3, p. 27
While skill bonuses are not allowed to be counted more than once, a rogue can expertly double the benefit of his skill bonus to two level 1 (and thievery) and again level 6 skills.
Backgrounds that provide redundant capability allow you to choose a different skill instead
The Backgrounds & PHB Background section lists the following rule:
If a character would acquire the same ability from two different sources, he or she can instead choose another ability of the same kind (ability or tool).
(Note that this rule may be unclear just apply for background knowledge.)
Custom backgrounds are also an option for players to avoid duplicate abilities due to backgrounds.