SPARQL query returns multiple birth dates for same person - sparql

Question

I am learning SPARQL and dbpedia by working through the queries in https://www.joe0.com/2014/09/22/how-to-use-sparql-to-query-dbpedia-and-freebase/ . I am testing a query to return John Lennon's date of birth and I am running my queries in http://dbpedia.org/sparql . The query is:
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX dbpedia-owl: <http://dbpedia.org/ontology/>
SELECT DISTINCT ?x0 ?x1 WHERE {
?x0 rdf:type foaf:Person.
?x0 rdfs:label "John Lennon"#en.
?x0 dbpedia-owl:birthDate ?x1.
}
It returns two rows containing the same date (9 Oct 1940). My question is: why does the query return two rows even though it uses DISTINCT? Prior to asking this question I checked the following:
Why does my SPARQL query duplicate results?
Duplicate rows when making SPARQL queries
but I don't think they explain the duplicate dates.
Edit: I converted the results to text and pasted them below
-------------------------------------- -----------------------------------------------------
x0 x1
--------------------------------------- -----------------------------------------------------
http://dbpedia.org/resource/John_Lennon 1940-10-09
http://dbpedia.org/resource/John_Lennon "1940-10-9"^^<http://www.w3.org/2001/XMLSchema#date>

Answer 1

As stated it seems dbpedia actually has two dates, 1940-10-09 (valid) and 1940-10-9 (invalid). The answer is to add a FILTER that converts the date to a string and only allows dates conforming to YYYY-MM-DD. Anyway it works!
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX dbpedia-owl: <http://dbpedia.org/ontology/>
SELECT DISTINCT ?x0 ?x1 STR(?x1) WHERE {
?x0 rdf:type foaf:Person.
?x0 rdfs:label "John Lennon"#en.
?x0 dbpedia-owl:birthDate ?x1.
FILTER (REGEX(STR(?x1),"[0-9]{4}-[0-9]{2}-[0-9]{2}")).
}

Answer 2

Well, it is not your fault! Simply the resource has both of these triples as you can see here. There are duplicates in the data.

Answer 3

I ran your query on the DBpedia endpoint and asked for the results in an RDF-based format (Turtle), and found that the lexical forms of the date literals are actually different:
"1940-10-09"^^xsd:date
"1940-10-9"^^xsd:date
The second isn't actually a legal xsd:date. The first is, which is probably why the SPARQL endpoint prints it in "pretty" fashion in the HTML table (as just 1940-10-09).

Answer 4

The result is a slowdown on queries because each access to an invalid date trig an exception (for example, with a query from fuseki) or the filter do the job to eliminate the wrong date, but it's costly