The attribute alias in the keystore configuration of undertow.xml seems to be ignored:

...
<w:keystore path="${karaf.etc}/../../server.keystore" provider="JKS" alias="antonio"
keystore-password="password" key-password="password"/>
</w:ssl>
</w:server-identities>

Looking at the xsd [1] the use of the alias should exclude all remaining entries in the keystore.

"The alias of the entry to use from the keystore, if specified all remaining entries in the keystore will be ignored.
Note: The use of aliases is only available for JKS based stores, for other store types this will be ignored."

But with the alias configured and a JKS keystore with two PrivateKeyEntry, only the first entry is used.