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Same issue for me (noobie as well. Had pfSense some time ago, but also far from Pro there).

So, double entries on Leases page for static-assigned devices. IP is the same, MAC is also the same (so it's not the case of LAN vs WLAN interfaces, or 2.4G WLAN vs 5G WLAN).
And static assignments are not overlapping with DHCP range. I think, OPNsense even wouldn't allow this.