Trailing slash or no trailing slash does not affect rankings. Serving both without redirecting between them does, because it splits the same page into two addresses.
What splitting costs
Links arriving at both forms divide the signals that should accumulate on one. Analytics reports the page twice. Any report comparing crawl data against traffic data shows false mismatches.
Decide, then redirect
Pick either convention and 301 the other form to it, site-wide. Which you pick genuinely does not matter; having both return 200 does.
Normalise historical data too
Existing exports, redirect maps and tracking configurations will contain the old mixture. Normalising them to the chosen form is what makes past and future data comparable, and it is the step most people skip after fixing the live site.
Try it: Remove Trailing Slash from URLs on SeoWolf's Notepad.