Screening of material from a film finally is repeated in French (4,4)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

A, (fil}M [finally], EST (is…in French) + EST [repeated]. Collins: a method of preliminary screening for carcinogens, based on their ability to cause mutations in bacteria named after Bruce Ames (born 1928), US biochemist. NHO this and the wordplay was tricky so I was only able to guess at the answer once all the checkers were in place. Seems a bit obscure to me.