How To Prevent Duplicating Records When Importing Data

We are a travel company and I am just setting up a new database with two tables - [Client_Table] & [Enquiry_Table].

Most exisiting clients call in when they want to make a new enquiry so the 'user' can go it to a form which creates a 'new enquiry' for that client.

We also download 'new enquiries' from our website. This data includes info that goes in to the [Client_Table] and [Enquiry_Table]. They download in to a XL spreadsheet. Currently I copy and paste the data (not sure if there's a better way) from the XL speadsheet in to a query. This query creates a new client record & a new enquiry record which are both linked by a primary key [Client_ID].

This all works however the problem I have is that sometimes existing clients enquire through the website. They do fill in a field to say they're an existing client but if I paste them in to the query as explained above it creates a duplicate client record.

The only way around this I can think of is it to take out any exisitng clients from the XL spreadsheet first, search for their Client_ID and then paste these enquiries seperately with their Client_Id's in to different query which only adds a new record to the [Enquiry_Table] and links them to their exisitng record in the [Client_Table].