My rear diff has a slight leak around the driveshafts

I have checked the breather and cleaned but looks like i need to replace the seals

I think I heard that there are 2 oil seals per side. 1 on the driveshaft and 1 on the cover plate. Is this true? Is it worth replacing both seals at the same time?

Are they main agent parts only or are the ones from my local motorfactors ok?

Any info appreiciated

Thanks
Scott