It usually creeps in over time: multiple blog posts on near-identical topics, an old and a new page targeting the same term, or product and category pages overlapping. Google may struggle to pick which to rank, links and relevance get diluted across both, and rankings become unstable.
Spotting it means reviewing which URLs rank for each key query (Search Console helps) and finding terms where several of your own pages appear and swap places. Overlaps often surface during content gap analysis.
Fixes depend on the case: consolidate two weak pages into one strong one with a 301 redirect, use canonicals to pick a primary, re-target one page to a different intent, or adjust internal links and anchors. The principle is one clear page per intent — which also keeps evergreen content tidy.