removing duplicated SNPs vs merging SNPS

Instead of removing SNPs, we can merge the bfile with itself
https://www.biostars.org/p/335304/

```
plink2 --bfile $outprefix.step1c --set-all-var-ids '@:#_$r_$a' --make-bed --out $outprefix.step1d

plink --bfile $outprefix.step1d --bmerge $outprefix.step1d --make-bed --out $outprefix.step1e
```