if you have different Seurat objects and the sample are processed in different batches, you may have cells with the same barcode across samples. You may need to prefix the barcode with the sample name.

for (i in names(seurat_object_list)) {
  seurat_object_list[[i]] <- RenameCells(seurat_object_list[[i]],
                                         add.cell.id = i)
}

# merge all the objects in the list
merged_combined <- reduce(seurat_object_list,
                          MergeSeurat,
                          do.normalize = FALSE)

or you can use https://samuel-marsh.github.io/scCustomize/reference/Merge_Seurat_List.html