And a second time they said, "HALLELUJAH!
Praise to the One Who Is -- twice indicates this is the highest honor we can give for our salvation (verse 1). Additionally, this time the PRAISE is for the complete victory over Babylon. No longer are these Babylonian practices (primarily economic) found within the body of believers. Instead the ones who removed themselves from Babylon (Revelation 18:4-5) are totally committed to living by faith (Luke 18:1-8).
Her SMOKE rises up FOREVER and EVER."
This confirms that Babylon, global economy and politics, are completely destroyed. The "ANY LONGER"s in Revelation 18:21-23 are permanent no matter how the devil and beasts will try to survive. The prophesy in Isaiah 34:5-10-15, if you believe that it is talking about this time, says there aren't any noblemen (economists or politicians) who can put whole system back together again in a recognizable form. The earth has been prepared for the 1000 year reign of Christ (Revelation 20:4-6).
SMOKE is all that is left of what was thought to be of substance. As such, it is a symbol of complete destruction and previously just the sight of it caused mourning (Revelation 18:9 and Revelation 18:18). What a contrast here, though, the sight of the SMOKE is cause for a burst of PRAISE!
Note that the duration, FOREVER and EVER, is the same amount of time that those who took the mark of the beast will be tormented (Revelation 14:9-12).