How do you find which two apps are conflicting?

We bisect. On the duplicate theme we disable apps one at a time while watching the browser console and network tab. When the error disappears, we have one suspect; we re-enable and isolate its partner the same way until the exact clashing pair is pinned. We also read the script load order, many conflicts are a race where whichever script loads second wins, so the fix is ordering, not removal.