Vistors might find it confusing; do you think levislevis.com makes as much sense as levis.com? Does footballfootball.com make perfect sense? I'm not so sure.
Also, no-one is very likely to search for domaindomain, so that one is less than ideal.
So in my view, it isn't the SEs you should be thinking about ... it's people.
For example, if you can't get wow.dom, wowwow.dom is not a good choice, but doublewow.dom would be more memorable and perhaps marketable too. Your site is not just Wow!--it's doubleWow!
Can't get threat.dom, how about triplethreat.dom?
Use a natural multiplier rather than a repetitive word.
Only get the multiple word version (exampleexample.dom or threatthreatthreat.dom) to redirect to the other site, in case people hear "doubleexample.dom" and type in exampleexample.dom instead.
Think pepsimax, cokezero - it's the 'core' of the name that carries the value - and if you tried too hard to commandeer it, you'd risk legal action - and / or dupe problems.
starbucksgold - looks like starbucks to me!
I still reckon unique is the way to go; build your own brand, don't parasitize someone else's; long term you'll always be a 'me too', never a success of your own. Short term, you'll lose all your repeat visiters to the 'home site' - domain.com
But I can't resist. I'm giddy. Don't expect a great deal of commercial value, but I registered some "fun" domain names as a result of this thread.
They revolve around that annoying annoying TV commercial...