A semi-duplicate is an article or other page that has a significant overlap with another article or other page or is otherwise a topic that would expect to be combined in 1 article. A semi-duplicate is distinct from a full duplicate in that a full duplicate is on exactly the same thing such as petrol/gasoline where the different names refer to exactly the same thing. Generally full duplicates are created accidentally when people don't realize that an article already exists while a semi-duplicate is usually created deliberately, that is to say they think a distinction should be made even though it probably shouldn't. Lsjbot created many semi-duplicates on the Cebuano and Swedish Wikipedias for example creating separate articles for municipalities in Italy with the same name as a settlement even though it is normal for such topics to be combined into a single article. If such articles do exist they tend to become content forks of each other however the term content fork can refer to a descriptive title that is split from the main article to represent a particular point of view (otherwise it may be argued this page is itself a semi-duplicate). Although Wikipedia is not a dictionary it is usually desirable to combine certain topics that have the same name and overlap.

• Low level municipalities such as civil parishes and comuni for example with Hatfield Peverel[1][2] and Campora[3][4] that have the same name as a settlement, the municipalities include a larger area/population but the boundaries of such units tend to remain stable for long periods and correspond to natural boundaries. Thus there isn't Hatfield Peverel and Hatfield Peverel (parish) or Campora and Campora (comune), see Wikipedia:Separate articles for administrative divisions to settlements. For higher level divisions such as Braunau am Inn/Braunau am Inn District generally separate articles should exist.
• Islands that are largely coterminous with administrative units such as Madagascar there is no Madagascar (island), see User:Seav/Islands and administrative units. However Ireland/Republic of Ireland have separate articles as the boundaries are significantly different.
• Territories that have different statuses or have changed boundaries such as Essex deals with both the ceremonial and administrative county and thus there is no Essex (administrative county), see Wikipedia:WikiProject UK geography/How to write about counties. Shropshire (district) however has a separate article as it's a district of the county rather the administrative county merely containing some of its districts.
• People only notable for 1 event where the event is named after them such as "Killing of X" since although the articles use a longer title than the main subject they will tend to discuss other people involved however most of the information will be for the main person Madeleine McCann redirects to Disappearance of Madeleine McCann. If there is a significant amount of information about the person separate articles may be justified though like George Floyd/Murder of George Floyd.
• DAB pages with titles that only differ by capitalization, articles, diacritics or pluralization, see WP:DABCOMBINE The Cure (disambiguation) redirects to Cure (disambiguation), however in some cases (normally pluralization) it may be desirable to have separate DAB pages if the singular and plural don't have much overlap or there are a large number of entries such as Friend (disambiguation) and Friends (disambiguation).
• Monotypic taxa, those with only 1 sub group only have 1 article, thus Nodocephalosaurus kirtlandensis redirects to Nodocephalosaurus, see WP:MONOTYPICTAXA.
• Legislatures of 3rd and 4th order administrative divisions apart from where the division is combined with another entity, Wyre Borough Council redirects to the district but Eastbourne Borough Council is separate because the district doesn't have a separate article from the settlement, see Wikipedia:Notability (councils).

If only a small number of separate articles exist then boldly merging or tagging them for merging is probably appropriate. If a large number of articles exist you should probably discuss at a Wikiproject or a RFC to decide if articles should make a distinction.

Note that speedy deletion criteria A10 cannot be applied to semi-duplicates. If the title of a semi-duplicate is considered problematic it may be appropriate to PROD/AFD the articles or merge to the correct title and send the demi-duplicate title to RFD though sending article content to RFD is controversial, some feel such articles should be restored and sent to AFD.

If you merge a complete duplicate the Wikidata items should normally also be merged, see d:Help:Merge however Wikidata may make a distinction for what we would not normally so care must be taken if its also standard on Wikidata.

The difference between content forking and semi-duplicates is, the content that is being forked that shouldn't be in a particular article, such as if there was a significant amount of information about apples in the Orange (fruit) article the correct thing would be to move the content about apples into the Apple article rather than merge Orange (fruit) to Apple. On the other hand there may be separate articles for a settlement and low level municipality even though the separate article for the municipality does not duplicate much content in the settlement article for example it would still be a semi-duplicate. Generally per WP:BEFORE C1 if there are problems with content forking they should be dealt with through normal editing rather than merging the articles. Similarly, just because the articles don't have content forking problems due to good editing doesn't mean the articles should be kept separate, similar to Wikipedia:No amount of editing can overcome a lack of notability. However, semi-duplicates have a higher risk of content forking though the article may be less likely to be merged if distinct content has been added that may justify an exception.

The difference between notability and semi-duplicates is, notability normally concerns substantially distinct topics, such as a settlement vs. primary school, where the primary school may be merged with the settlement because it's not notable enough for a standalone article. If separate articles exist for the settlement and primary school it's unlikely they would duplicate much but the school may not pass the notability guidelines. On the other hand, with a settlement v s.low level municipality the municipality would clearly pass the notability criteria as with the settlement but such topics overlap too much to have different articles.