(actually tested with git 5.1 branch) When a user buys an existing security in an Investment Account which does not already hold that security, KMM does not recognize the security ("The security Security Name currently does not exist as sub-account of Invest Account. Do you want to create it?") and then starts the New Investment Wizard.  To reusing the existing security, the user has to know to hit tab, delete the Full name, hit Shit-Tab, enter the existing Trading Symbol, hit Tab, at which point KMM prompts to reuse the existing security.

I can think of several possible approaches for this.

Currently, the Security dropdown in a new Investment transaction shows only the securities already held in that Investment Account.  One possibility would be to include (probably after some divider) those securities not already held in that Investment Account.  Another would be that if the name typed in is not already held in that Investment Account, to check if it does exist as a security, and if so, prompt the user whether the existing security should be reused.

An easier interim solution might be just to add some static text near the top of the New Investment Wizard, Investment details page.  Right above the Trading symbol, we could add something like: "If you want to reuse an existing security which is not already held in this Investment Account, then clear the Full Name, then enter the Trading Symbol and hit Tab or select another field."

A further option would be to test if the Full name already exists, not just the Trading Symbol.  A variant on that approach would be to test if the Trading Symbol already exists, even if the Full Name is filled in on that page.