Yeah, It was never recommended to buy a kit and then expand upon it afterwards, although it is possible, chances are you wont get the performance or stability you're expecting, even if the kits you buy separately are identical. I didnt know this back in the day and I wanted to upgrade from 4gb of Ram to 8gb after upgrading to a x64 OS . I bought the exact same 4gb I had in my PC thinking it would work and damn was it a real headache to get the 4 sticks working together at all. Had many instability issues along the way.
Nowadays its better to just outright buy a 16gb kit for example, than it is to buy an 8gb kit and upgrade later by buying another 8. You're just asking for trouble! Most people would suggesting buying an entirely new kit of a higher capacity.
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