It's a little cumbersome, but here is a solution. You have your primary table, with recordA and recordB, and that table has your date field. See "Table1" below.
Add a secondary table and prefill it with all possible dates as the primary key (you can do this quickly in Excel and then paste in Airtable). Then add a linked field and allow multiple links back to the other table. Add a Rollup field to the secondary table & use COUNTALL(values) as the formula in the linked field. Create a view on the secondary table that filters to where the rollup value is >1.
Then add two automations - the first is associated with Table1. The trigger should be when the datefield is updated, to update the linked field column with the same string (which will link if string matches the PK in other table).
The second automation is for your alert email in Table 2. If any record enters your >1 view, then fire an email.
Table1

Table2
