How it happens
Usually by accident: first you wrote an article about a service, then created a landing page, but did not remove the old one. Both are about the same thing, both fight for the same query.
How to spot it
A sign – rankings for the query fluctuate, and the search results show either one page or the other. You can check it by searching your own site: type the query in the search engine and add site: with the address.
What to do
Decide which page is the main one, and either rewrite the second one for a different query, or merge it with the first one using a redirect. You should not delete it without a redirect – you will lose the accumulated signals.