This problem occurs when importing a new QIF security. If on the commodity page of the druid you then choose a combination of exchange and symbol that already exists in GnuCash, the import will fail.

Steps to reproduce:
1. Begin importing a QIF that uses a never-before-seen security (i.e. one not present in .gnucash/qif-accounts-map).
2. When the importer displays a commodity page for that security, enter a combination of exchange and symbol that is already used in an existing GnuCash commodity.
3. Attempt to finish the import.

Actual results:
The import fails with a generic pop-up error message.

Expected results:
The importer doesn't create a new commodity. The existing GnuCash commodity is used for the import.