Dates with precision lower than month (10), mainly dates with year precision (9) - but also century (7) and others -, could be encoded in different ways, thus falsely duplicating values and generating single-value constraint violations. This is a known problem also affecting the use of QuickStatements (https://www.wikidata.org/w/index.php?title=Help:QuickStatements&oldid=1657403335#Removing_statements), whose users should take into account both formats instead of one of them.
For year precision (9): yyyy-01-01 vs yyyy-00-00
* https://www.wikidata.org/w/index.php?title=Q65174527&oldid=1663810999
* https://www.wikidata.org/w/index.php?title=Q60391412&oldid=1663738709
* https://www.wikidata.org/w/index.php?title=Q63869992&oldid=1663779113
Other documentation:
* a discussion at the German project chat (2016): https://www.wikidata.org/wiki/Wikidata:Forum/Archiv/2016/11#Angabe_von_Zeitpunkten_in_Jahresgenauigkeit
* a discussion at the main project chat (2017): https://www.wikidata.org/wiki/Wikidata:Project_chat/Archive/2017/06#Merge_tool_doesn't_merge_dates_as_expected
* one non-participated RfC (2018): https://www.wikidata.org/wiki/Wikidata:Requests_for_comment/1983-01-01_date_of_birth_vs._1988-00-00_date_of_birth_(both_only_have_a_year_specified): "there are different ways of inserting the date and the software does not enforce the only format"
* one non-fulfilled bot request (2020): https://www.wikidata.org/wiki/Wikidata:Bot_requests/Archive/2020/05#Change_year_precision_dates_from_\d\d\d\d-00-00_to_\d\d\d\d-01-01_(notably_P569,_P570): it asked for using 01-01, but noted that manual entries use by default 00-00
* a discussion at the Czech project chat (2021): https://www.wikidata.org/wiki/Wikidata:Mezi_bajty/Archive/2021#Přesnost_na_rok_-_vkládat_01-01_nebo_00-00?
* another bot request (2021): https://www.wikidata.org/wiki/Wikidata:Bot_requests/Archive/2021/11#request_to_automate_marking_preferred_rank_for_full_dates._(2021-05-28)
As of now, MatSuBot (operated by @matej_suchanek) [[ https://github.com/matejsuchanek/pywikibot-scripts/blob/master/cleanup_dates.py | merges claims ]] just differing for 00-00 and 01-01 preferring the claim with sources in order to perform a smaller number of edits; it looks for claims to be merged through a [[ https://github.com/matejsuchanek/pywikibot-scripts/blob/master/queries/unmerged_dates.txt | query ]] (see https://www.wikidata.org/w/index.php?title=Topic:Wxspna7q8jnn17u8).
The proposal is: allowing only one format and uniforming all existing dates with precision lower than 10 to that format.