You mean another Saphire RX590? Then no, there won't be any conflicts. You can mix brands, as long as the GPU (chip) is the same.
Despite what may be different clockspeeds, mem speeds etc, it will downclock both in Xfire to the lowest common denominator. Whichever card is lower clocked.
As Lutfij has said though, ensuring you have a capable PSU to power the Xfire setup is important.
And I'd totally agree with Lutfij on the single v dual GPU's. Having the single best card you can afford is the way to go. Sell the RX590 and get a RTX2070 super, or RX5700xt. You will enjoy the experience more.
SLI/XFire aren't supported much anymore, and still have micro stutter issues. From the fun point of view and playing around have 2 GPU's is cool. But the downside being the scaling in games is crappy, and the whole fad is less supported now by game developers, and is on it's way out IMO