By default, an email is unique to a list and can be added only once, using opt-in forms, mass upload, api or third party integration. If you want to allow adding a contact, more then once to a list (ex: Contact us form on your website), navigate to your Lists page and from the right side options drop down choose Form Builder. Expand the Email Address field and mark it Not Required. 

Note: When you make your email Not Required, you will allow people to submit your form without providing their email address. You might want to add an input required attribute to your email form field where the form has been embedded.

View the modified embed form below.

<div class="vboutEmbedFormRow">
   <label class="title" for="custom-74068">Email Address</label>
   <div class="vboutEmbedFormField"><input required type="email" name="vbout_EmbedForm[field][74068]" id="custom-74068" value="" class="vfb-text   email email" data-error="Please enter your email." placeholder="Please enter your email." /></div>
</div>