Created a AAD-UserReadUsingEmailAddress-RaiseIfExists Technical Profile under AAD Claims Provider:

<TechnicalProfile Id="AAD-UserReadUsingEmailAddress-RaiseIfExists">
      <Metadata>
        <Item Key="Operation">Read</Item>
        <Item Key="RaiseErrorIfClaimsPrincipalDoesNotExist">false</Item>
      </Metadata>
      <IncludeInSso>false</IncludeInSso>
      <InputClaims>
        <InputClaim ClaimTypeReferenceId="email" PartnerClaimType="signInNames.emailAddress" Required="true" />
      </InputClaims>
      <OutputClaims>
        <!-- Required claims -->
        <OutputClaim ClaimTypeReferenceId="objectId" DefaultValue="NOTFOUND" />
        <OutputClaim ClaimTypeReferenceId="objectIdNotFound" DefaultValue="NOTFOUND" AlwaysUseDefaultValue="true" />
      </OutputClaims>
      <OutputClaimsTransformations>
        <OutputClaimsTransformation ReferenceId="AssertObjectIdObjectIdNotFoundAreEqual" />
      </OutputClaimsTransformations>
      <IncludeTechnicalProfile ReferenceId="AAD-Common" />
</TechnicalProfile>



The objectIdNotFound is a simple string claim:

 <ClaimType Id="objectIdNotFound">
    <DisplayName>Used for comparison</DisplayName>
    <DataType>string</DataType>
 </ClaimType>


The AssertObjectIdObjectIdNotFoundAreEqual OutputClaim Tranformation is as following:


   <ClaimsTransformation Id="AssertObjectIdObjectIdNotFoundAreEqual" TransformationMethod="AssertStringClaimsAreEqual">
    <InputClaims>
      <InputClaim ClaimTypeReferenceId="objectId" TransformationClaimType="inputClaim1" />
      <InputClaim ClaimTypeReferenceId="objectIdNotFound" TransformationClaimType="inputClaim2" />
    </InputClaims>
    <InputParameters>
      <InputParameter Id="stringComparison" DataType="string" Value="ordinalIgnoreCase" />
    </InputParameters>
  </ClaimsTransformation>


Then i used the AAD-UserReadUsingEmailAddress-RaiseIfExists as a Validation TechnicalProfile in my SelfAsserted TechnicalProfile

<TechnicalProfile Id="LocalAccountSignUpWithLogonEmail-CheckEmailAlreadyExists">
      <DisplayName>Email signup</DisplayName>
      <Protocol Name="Proprietary" Handler="Web.TPEngine.Providers.SelfAssertedAttributeProvider, Web.TPEngine, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />
      <Metadata>
        <Item Key="IpAddressClaimReferenceId">IpAddress</Item>
        <Item Key="ContentDefinitionReferenceId">api.localaccountsignup</Item>
        <Item Key="language.button_continue">Next</Item>
        <Item Key="UserMessageIfClaimsTransformationStringsAreNotEqual">There is another user with this email address</Item>
      </Metadata>
      <InputClaims>
        <InputClaim ClaimTypeReferenceId="email" />
      </InputClaims>
      <OutputClaims>
        <OutputClaim ClaimTypeReferenceId="email" PartnerClaimType="Verified.Email" Required="true" />
      </OutputClaims>
      <ValidationTechnicalProfiles>
        <ValidationTechnicalProfile ReferenceId="AAD-UserReadUsingEmailAddress-RaiseIfExists" />
      </ValidationTechnicalProfiles>
</TechnicalProfile>