Same movie has been released both on its own and as part of a series

Sort of.

See the guidelines for Shell Episodes here:

https://help.imdb.com/article/contribution/titles/episode-guidelines/GDF7HR6CCCBKU3CP?ref_=helpsrall#

• If a title is later re-released as an episode of a series, then it should be added as a shell episode. This means the episode can be added to a series, but no filmography credits relating to the original title should be added to the shell to avoid crediting the work twice on the name pages of those involved. If the individual contributed to new material relating solely to that episode e.g. hosting segment then they are valid credits and may be added to the shell episode). Please also add a trivia item and a movie connection linking the episode to the original title page.