Hello les amis !
What is your best tip to dedupe (without using the extension of course) 
?
I’ve used custom scripts to tag potential duplicates. I usually prefer to have a human resolve the duplicates versus automatic deducing.
Why don’t you want to use the Dedupe extension? It works well and is available in all plans (except legacy free plans).
Welcome to the community, @CelineFromFrance!
Hmmm… deduping without the extension is definitely a tricky one! 
As @Kuovonne mentioned, scripting can be used for this.
Outside of scripting, I’ve created duplicate-checking & deduping automations for my clients using Make.
Also, if you just wanted to look for duplicates within a small number of records that are linked to a unifying record in another table— such as duplicate students on a field trip, or duplicate line items on an order — you can “sort of” check for duplicates. You could use a rollup field & formula field to alert the user to the presence of a duplicate somewhere within the set of linked records, but it wouldn’t let them know exactly which record is the duplicate.
Thank you so much for your responses. I had to dedupe 3000 records, but that is true @Kuovonne, no automation could do that for me because there were human decisions to make 
I use the extension, could have been worst on excel 