merge securities with same name and symbol?

sleon said:

There are several message threads reporting no easy way to do this. Has Quicken figured out and delivered a tool that makes this easy?...

You are right.  There are several discussions about this issue.  There is no 'easy' way to do that.  Quicken has not implemented a way to make that easy.  You have to make the correction the 'hard' way as undoubtedly presented in those several discussions.  

sleon said:

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How do I merge all the history for one of the entries into the history of the other entry.
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The hard way - Edit security names in individual transactions as needed.

sleon said:

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And, how do I prevent Quicken from creating two separate/different entries for the same stock?
In advance, much and many thanks.

Pay attention.  Commonly, when a brokerage download reports to Quicken a new security in the account, Quicken will prompt the user to identify whether that new security is truly new within Quicken or if should it be matched to an existing security.  Quicken will usually make a guess based on ticker and name and bias the prompt one way or the other, but that guess and bias are not foolproof.  If the user too quickly accepts or chooses the New security option, the file can end up with similar Quicken securities representing the same real world security.  (It is possible that in less common circumstances not fully identified, Quicken might blow by that pop up prompt and unilaterally create a new security, such as now exists in your case.)

It is an absolute that the Quicken security with the CUSIP is matched to the real world security your brokerage reports.  The non-CUSIP security almost certainly was user-generated.