SPARQL - Speechnumber with earliest start date for party membership - date

Question

I am looking for all speeches with certain dictionary terms in a particular time frame.
For this, I want name, country, birth, gender, partyname, start date for joining the party, date of speech, and speechnr. In the output however, I get different start versions for the same speechnr. I only want the earliest version (i.e. when they first joined the party). How do I do this?
Here is my code so far:
SELECT DISTINCT ?name ?countryname ?birth ?gender ?partyname ?start ?date ?speechnr ?parlterm ?dictionary
WHERE {
?speech lpv:spokenText ?text.
?speech dcterms:date ?date.
?speech lpv:docno ?speechnr.
?speech lpv:speaker ?speaker.
?speaker lpv:name ?name.
?speaker lpv:dateOfBirth ?birth.
?speaker lpv:gender ?gender.
?speaker lpv:politicalFunction ?function.
?function lpv:institution ?party.
?party rdf:type lpv:NationalParty.
?party rdfs:label ?partyname.
?function lpv:beginning ?start.
?speaker lpv:countryOfRepresentation ?country.
?country rdfs:label ?countryname.
BIND("8" as ?parlterm)
BIND("domestic violence" as ?dictionary)
FILTER ( ?date > "2014-07-01"^^xsd:date )
FILTER(langMatches(lang(?text), "en"))
FILTER(CONTAINS(?text, 'domestic abuse') || CONTAINS(?text, 'domestic violence') || CONTAINS(?text, 'intimate partner violence') || CONTAINS(?text, 'rape') || CONTAINS(?text, 'sexual violence') || CONTAINS(?text, 'sexual assault') || CONTAINS(?text, 'gender base violence') || CONTAINS(?text, 'gender-based violence') || CONTAINS(?text, 'violence against women') || CONTAINS(?text, 'femicide') || CONTAINS(?text, 'witch-hunting death') || CONTAINS(?text, 'femicide-suicide') || CONTAINS(?text, 'excess female mortality'))
FILTER NOT EXISTS {
?function lpv:beginning ?start2
FILTER (?start2 < ?start)
}
} ORDER BY ?date ?speechnr
As you can see, I tried to add a Filter Not exists at the end but it doesn't change anything. I would be really glad for help.
Here is the data for anyone interested:
https://linkedpolitics.project.cwi.nl/web/html/home.html