This lookup-and-alert can't be done with the standard platform.
You can ask support to have your dedupe rules include this field, but that would mean the leads are merged (you could tell when the 2nd+ forms had been submitted, though).
A webhook engine like FlowBoost (Pro) can do this, by performing a vertical lookup across stored data.
Marketo's ABM add-on is another option.
Thanks for your answer, yea unfortunately what I thought the answer would be! No perfect solution at the moment.
I will most likely use Zapier to send field data to spreadsheet to do the calculation + send the alert.