Edit the customer & change the name to exactly match the one you want to keep.  When you click save, it will say ‘this name is already being used, do you want to merge them?’ — click yes.  Be careful — there is NO undo.

Resolution for 'combine duplicate issue: 'How do I combine duplicate customers?' available: Yes.

The combine duplicate issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the combine duplicate issue yourself by looking for a resolution described in the article. If it is a complex combine duplicate issue or you are unable to solve the issue, you may contact us by clicking here or by using other combine duplicate support options.

For more information about resolving issues related to combine duplicate in Quickbooks/QBO or to get support for the issue 'How do I combine duplicate customers?', click the link below: