Because two e-mails from two different accounts are being sent to the Lansweeper helpdesk e-mail account, it is indeed normal for Lansweeper to create two tickets. The only way to stop Lansweeper from creating two tickets would be to create a filter in Configuration\E-mail Settings, which ignores your personal e-mail address. That way when the user replies to your reply, the ticket will be created.
Alternatively, you could create ticket dispatching rules which set e-mails coming from your personal e-mail address to a specific team, state or type.
The recommended way of working would be to ask users to not e-mail you personally, but e-mail the Lansweeper helpdesk e-mail address.