I recently got my '84's duke going for the first time in 24 years, and then it had a high idle, starting at 3 and increasing to around 4k before I turned it off. I replaced most of the vaccum lines and now it starts at 2k and slowly increases up to around 3-3.5k after around 3 minutes. Fixing the lines seemed to fix a lot of the high idle, but I'm not sure what could fix an increasing idle. Any ideas? Thanks!

The EGR valve could be stuck open, the EGR gasket to the manifold could be leaking, the diaphragm could be leaking or the hose from the EGR to the TB is bad. I had the same problem in two Fieros and just removed the EGR valve and blocked it out completely.

See my Cave, Vac Leaks

But note: If you can't/won't drive the car... likely had to replace battery and ECM needs to "reset" somethings and will have high idle until drive > 35mph for a few blocks minimum.

Does idle drop some when bump the gas? Unless very cold to freezing weather should slow down some but will often still have to Reset before you see normal idle.

If weather is cold/freezing then won't see normal idle w/ ECT seeing cold coolant. If ECT or wires to it are iffy then tell bogus coolant temp to ECM does same thing. Need ECM scanner to really see sensor data.

Car not run in months to years then have other problems now or soon.
Example: Water Pump seal is likely to fail because no/weak coolant and rust on shaft eats it. Good Coolant is lube for the seal and anti rot for just about everything else. Car sitting years or decades means no protection except glycol keeping system from freezing.

Likely needs brake fluid and other brake problems fix. See rest of cave for coolant and brakes.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave