If when contacting the enquirer it is found that they are already an existing customer, you should link this Enquiry to their Customer Contact record.

You can check if they are an existing customer by using the global search:

As you can see Ryan has an existing Cust. Contact record and an enquiry record.

We recommend you go to their Cust. Contact record and create and/or link the enquiry to that record. This way any of the information the individual has already provided can be used to streamline the process.