Let's call your data frame df.
For your request to make sense, densities has to be constant for a given combination of quadrat and year. You could do any of the following.
dfNew <- df %>% select(quadrat, annee, fusion, densite ) %>%
unique()
#Keeping fusion is redundant.
dfNew <- df %>% select(quadrat, annee, densite ) %>%
unique()
#Could also use median(), densite is constant
dfNew <- df %>% group_by(quadrat, annee, fusion) %>%
summarize(DensiteAvg = mean(densite)
#Again, fusion is redundant
dfNew <- df %>% group_by(quadrat, annee) %>%
summarize(DensiteAvg = mean(densite)