I am playing a rogue that I built a little over a year ago for my first ever game of D&D. We play about once a month or so in an ongoing campaign. My DM set my ability scores for me when we began, because I had no idea what I was doing at the time!
I could use an extra point to my wisdom, 13→14. My strength is a dump stat at 10 and doesn't need the boost. (We did 5d6 best 3 for scores.) However, I would like to gain proficiency in Strength saving throws. I want proficiency in Wisdom saving throws too, but that will come once I get Slippery Mind at Rogue level 15 (PHB p. 96).
My question is: can I take the Resilient feat (PHB p. 168) now, to gain that extra point in Wisdom and proficiency in Wisdom saves, and then once Slippery Mind comes to grant me Wisdom save proficiency again, pick another ability of my choice to gain a save proficiency in instead? The PHB says that skills and tools work this way, but doesn't mention this for saves. (There aren't any conflicting ways to gain save proficiencies other than the Resilient feat as far as I know.)
I'd love to know if there's an official rule for this, but if not, I may just have a case to make to our DM.
You don't get to choose a different save proficiency
The rule you're thinking of that lets you choose a different skill or tool proficiency if you gain a proficiency you already have isn't a general rule, it's a rule only for proficiencies gained from Backgrounds. The rules is on page 125 of the PHB, and is part of the rules on how to apply the benefits offered by Backgrounds. In addition, Backgrounds are only available at level 1 during character creation, so that rule will never apply during later character advancement choices.
Since Slippery Mind and Resilient aren't Backgrounds, if you gain a redundant proficiency from one of them, that rule doesn't apply. And since there's no similar general rule that lets you choose a different proficiency when gained from a non-Background source, getting to choose an alternative proficiency isn't something the game lets you do.
Unfortunately, you'd just gain proficiency in Wisdom again, which would grant the character nothing new.