Shimano Claris shifter double/triple shifting up (when moving to smaller, harder cogs) on the rear derailleur
Check the following:


If you remove the cable tension, do you get the right number of clicks for the shifter? If not, you may have a broken pawl or gummed up pawl. If blasting with wd-40 doesn't work, you need to get a new shifter, cause taking apart a brifter and putting it back again is hard.
Is the derailleur bent? It isn't possible to adjust a bent derailleur satisfactorially. To unbend it properly, you need a special tool. 
Is the chain worn? If so, get a new chain. Check with a gauge. 
Is the cassette worn? If so, get a new cassette. And probably a new chain, for good measure. See this link on chain + cassette wear. 
Is the derailleur adjusted properly? The cables do slip over time, and you may need to readjust them. Follow the directions here. 
Is the cable slipping or have excess friction or under too much or too little tension? You can clamp down the cable and/or replace it. You can also get weird things if your frame flexes enough to pull the cable, e.g. if you have under-bottom bracket shift cable routing.