I have found that a website which appears above mine for a certain popular keyword combination has duplicated the Description meta tag.
Is this bad practice? Is it something that Google might notice?
If it’s vague marketese, as it seems from the symptoms, there is nothing you can do against copying. Google won’t care.
If they use your company name or trademarks without permission, you might consider sueing them. It will cost a lot of money, and it is rather uncertain who would pay the bill. Google won’t care.
They should, just you should, create an unique title and description for each page because you get points for that. This sounds pretty, but might be very tricky to do in larger, more automated websites (possible the one you are refering too).
However, this is merely one of the dozens of options you have to check in order to increase your page rank (eg: proper H1's, valid html, ...), of which the two most important are easily number of visits and duration on site which both can be achieved with good content.
If they have those two, and you don't, they dont need to mind the duplicate pages, they simple out-PR you.