Shimano Claris shifter double/triple shifting up (when moving to smaller, harder cogs) on the rear derailleur Although my rear derailleur is properly indexed it's suddenly started to double or even triple shift on an upshift - i.e. when moving to harder, smaller cogs on the cassette.

This happens whether I'm riding or even if the bike is completely stationary and I'm only moving the shifter in isolation - I can feel and see the display on the hood jump 2-3 positions.  It only happens in what would be the mid positions of the cassette - the very bottom gear and the top few gears aren't affected when I shift up from them.

Downshifts to larger, easier cogs are completely unaffected.

I've tried peeling back the brake hood and giving the mechanism a good blast of penetrating lubricant but to no avail.

Is this likely to be a shifter only problem and what if anything can I do to rectify it?
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.