Level 2

Extension request mistakenly filed AFTER tax already filed?

My newly hired accountant e-filed my 2018 IRS tax return on Apr 12. My old accountant who doesn't know I no longer want their service, just e-filed IRS extension for me on Apr 15, expecting me to use their service to file in the future.
Will this bring any trouble?
I don't know IRS system will accept any extension request when a tax return is already filed (maybe even the extension request is not allowed to be E-filed in this  case?). If it really accepts it, will this wrong extension invalidate previously filed return so I have to refile in the future?

If I receive my refund several weeks later, I can ensure IRS has disregarded the extension?