[SalesForce] Having problems passing a variable and displaying a Message

I'm new to VisualForce and to Java,

I have a custom object I created called "Inquery__c"
And I have a few users on record,

I'm trying to allow only the users that exist in the "Inquery__c" to upload a file to Documents,

Here is the code

<apex:page controller="newCheck_and_upload" tabStyle="Opportunity">
<apex:sectionHeader title="Please Enter user information and upload LogFile"/>
<apex:messages /> 
<apex:form enctype="multipart/form-data">
   <apex:pageBlock>
      <apex:facet name="footer">
      <apex:outputPanel >
         <apex:commandButton action="{!save}" value="Check and Uplod" styleClass="btn"/>
      </apex:outputPanel>
      </apex:facet>
   <apex:pageBlockSection title="User Information">
      <apex:panelGrid columns="2">
         <apex:outputLabel value="First Name" for="inqueryFirstName"/>
         <apex:inputField id="inqueryFirstName" value="{!inquery.First_Name__c}"/>
         <apex:outputLabel value="Last Name" for="inqueryLastName"/>
         <apex:inputField id="inqueryLastName" value="{!inquery.Last_Name__c}"/>
         <apex:outputLabel value="Email" for="inqueryEmail"/>
         <apex:inputField id="inqueryEmail" value="{!inquery.Email__c}"/>
      </apex:panelGrid>
   </apex:pageBlockSection>
   <apex:pageBlockSection title="Upload File">
      <apex:panelGrid columns="2">
         <apex:outputLabel for="file"/>
         <apex:inputFile value="{!document.body}" filename="{!document.name}" id="file"/>
      </apex:panelGrid>
   </apex:pageBlockSection>
</apex:pageBlock>
</apex:form>
</apex:page>

And the Class

public  with sharing class newCheck_and_upload {

Inquery__c inquery;
public Boolean sFlag;

public Inquery__c getInquery() {
if(inquery == null) inquery = new Inquery__c();
   return inquery;
}

public Document document {
get {
  if (document == null)
    document = new Document();
  return document;
}
set;
}


 public PageReference save() {

     if(inquery == null) {
      sFlag = false;
     }else {
     sFlag = true;
     }  

 if(sFlag){   
     ApexPages.addMessage(new ApexPages.message(ApexPages.severity.INFO,'User Exists'));
    // sFlag = false;
    } else {
     ApexPages.addMessage(new ApexPages.message(ApexPages.severity.INFO,'User Does not exist'));
     //return null;
    }

      insert inquery;

    document.AuthorId = UserInfo.getUserId();
    document.FolderId = UserInfo.getUserId(); // put it in running user's folder

    try {
      insert document;
    } catch (DMLException e) {
      ApexPages.addMessage(new ApexPages.message(ApexPages.severity.ERROR,'Error uploading file'));
      return null;
    } finally {
      document.body = null; // clears the viewstate
      document = new Document();
    }
    ApexPages.addMessage(new ApexPages.message(ApexPages.severity.INFO,'File uploaded successfully'));

    return null;
 }

}

When I enter a user that exists or a user that does not exist (both ways) the file is never uploaded. This means that sFlag is never True.

When i try saving an existing file i get the following error:

System.DmlException: Insert failed. First exception on row 0; first error: DUPLICATE_VALUE, duplicate value found: Email__c duplicates value on record with id: a08K0000001XxV5: []

Error is in expression '{!save}' in component in page exist_upload.

.

This means that the "inquery == null" stament is not working,.

And it return null,.

But when it tries to save the account,.

it tells me that the account is a duplicate!!.

.

Does anyone know why?

Can someone please help?

Thanks