Is there a way for courses to appear are as "Completed" for users that have taken the course in the course catalog?

Unfortunately not directly without using some widgets as a workaround, but folks still get there. 
good news is an idea already exists:

https://community.docebo.com/ideas/show%2Dcompletion%2Dstatus%2Din%2Dcatalog%2D1634
 

bad news is it is parked and waiting for dev time. 

You could also filter out the ones you are already enrolled in so that only courses you have not taken are visible.

But manually and every time :(

Yeah - filters are one of those things could go either way.

Sometimes you want a filter to be persistent and other times you don’t.

It seems you’re always stuck either setting the filter/sort or unsetting it.

Like the default of sorting by most relevant on websites. LOL  Ugh

I think it would be nice to somehow implement some storing of user preferences on things.

@gstager thought we had somme ideas on just that but maybe it was for the admins..??

found this that might work if you build it as a custom page

@lrnlab - Yeah - an admin could make those widgets or links on a page for access by the users but users could not create one themselves.

agree @gstager ...a new page would need to be built with that filter always ON...but in the absence of a “saved search” or “saved filter” option for users this could be helpful. We use this with pre-filtered search results often and it works well.

The problem I’ve seen with that is while on the custom page is fine, but users inevitably end up on the default catalog type pages at some point and then are confused as to why things are different. Whish there was a way to essentially replace a default page type with a custom page….