How to remove duplicated genes from a count expression data frame by mean expression

Question

[BioQueen]

Hi! I have a gene expression data frame consisting of count data with rownames as ensembl ids and colnames as samples in addition to one column which consist of the gene symbols.

I need the gene symbols as rownames to further analysis, but when I try to make the gene symbol into the rownames then I get an error message that says that I have duplicate genes. I have tried to search a bit, but I'm new to programming in R. I have seen some that collapses the duplicates and used max, but I want to use the mean value. Is there anyone who know how I can do this in R?

Thanks a lot!