There is always lots of talk about how and where to keep your back up private keys, paper wallets, 2 of 3 paper wallets and so on but not many people talk about storing the 12 word mnemonic seed.
I understand why people may not want to talk about it but also many like myself up until recently didn't put much care into it.
I would open a new wallet monthly ( I like testing different wallets) write down the seen on a piece of paper and lock it up in a filling draw.
Some often got lost in the mess.
This is a good link to understanding the use and need for the seed:
https://www.google.com.au/url?sa=t&sour ... nvV57Qk6gw
Is it common for people to make copies of their mnemonic and put them in different locations?
Normally if you need your mnemonic it's because you have forgotten your password so it's probably a good idea to store them separately.