This issue may occur if the Forgot Password email template is still enabled in FusionAuth. To resolve this:

Disable the Forgot Password email template at the tenant level:
Navigate to Tenant > Edit Tenant > Email tab > Template Settings.
Set the Forgot Password template to Feature Disabled.

Check the application-specific settings:
If you have an application-specific template set up, ensure the Forgot Password template is also disabled under the Application settings.

Verify API configuration in the PHP client (if applicable):
If you’re using the PHP client, ensure sendForgotPasswordEmail is explicitly set to "false" (as a string with quotes), not just false (boolean). This ensures the value is passed correctly.
These steps should prevent FusionAuth from sending its own Forgot Password email when the API is called, resolving the double-email issue.

Generally this is a dev time message. Although depending upon your integration, it may be possible that an end user would see that message.

You could try adding a message to your theme:

[MultiTenantSSONotSupported]=n/a

In general, any user facing message can be overridden by your theme.