When you add new tags, two things can cause problems on your site: near-duplicate tags and tags that break an existing naming pattern.
Reuse existing tags exactly. A tag goes public as soon as a single listing carries it. A typo or a near-variant therefore becomes its own separate filter. This is how accounts end up with "Pool" and "Pools" side by side in the same filter list. Before you create a tag, check whether one already exists and reuse it exactly, rather than spelling it slightly differently.
Follow the naming pattern of your current tags. If your site displays a family of tags in a special format (for example, distance tags rendering as "Stratton Mountain: 6.1 mi"), that presentation depends on the tag name matching the established pattern. A new tag in the same family that is named differently still works as a filter, but may display raw instead of formatted.