Categories that have the same meaning should be combined through a soft-redirect (put the {{RedirectCategory}} template in that category that says all the content should be moved). Several reasons for category redirects:

• Plural/Singular (i.e. Category:Website and Category:Websites)
  • The main category names should always be singular¹
• Synonymous/related word/other name (i.e. Category:Authors and Category:Writers)
• Abbreviation (i.e. Category:UK and Category:United Kingdom)
  • The main category names should not be abbreviated, and if it does, it should be in all caps (e.g. CPR not Cpr)¹
• Other languages (Category names will be in English language, like Wikimedia Commons. See {{TranslationTable}})
• Other spelling (Category:US, or Category:USA)
• Other capitalization (i.e. Category:Wiki and Category:WIKI)*

  Rule of thumb: if it's not an entity's name, don't capitalize other than the first word.

• CamelCase/not (i.e. Category:WebSites and Category:Web Sites)*
  • The main category names should not be camelcased except if it's more widely known (e.g. it should be Websites, not Web Sites)¹

*Currently is auto-redirected by the engine. In the future it should not.
¹Subject to change. Exceptions may exist