Pasting symbols now creates unique symbols instead of instances of the original symbol
Copy-pasting a symbol--whether individually or by using the paste to all artboards command-- now creates a new symbol instead of creating instances of the original symbol. Alt-dragging the symbol to a new artboard creates a new instance of the original symbol as expected.
This seems to be a new bug, as I can't recall ever having this issue before. Thanks for checking into it!

Please accept my sincere apology for the inconvenience. Please get in touch with us at any of the other support channels – https://helpx.adobe.com/support.html . Since this is not a generic issue that we can reproduce at our end, we will need someone to look into your machine to figure out what is going on here.
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Eric Bryant commented
I am having this same issue
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Alexey Korepov commented
"life is pain" © :)
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Ricardo commented
it happens all the time... ALL THE TIME.. drag and droping or copy pasting via keyboard. It duplucates the simbol instead of the instance
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Alexey Korepov commented
I have same problem with fresh Illustrator version not every time, but randomly very often when copying symbols, here is thread about this: https://forums.adobe.com/message/10543855#10543855
Also here https://forums.adobe.com/thread/2368949 is same bug report.
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Neat commented
Update: so it appears the issue is isolated to one specific symbol in the file I'm working with. No idea what could be triggering it. The symbol in question is a header that contains an outlined version of a logo that I converted to a symbol and a dotted line. Nothing too extraordinary about either of them besides the logo symbol having a lot of components because there's a small illustration next to it.