Altium PCB importer generates duplicate warnings for layers

Description

Steps to reproduce

1. Import any altium PCB (see https://gitlab.com/kicad/code/kicad/-/wikis/Test-Boards for examples)
2. Notice a large number of duplicate errors are shown

KiCad Version

Application: KiCad PCB Editor x64 on x64

Version: 7.0.7, release build

Libraries:
	wxWidgets 3.2.2
	FreeType 2.12.1
	HarfBuzz 6.0.0
	FontConfig 2.14.1
	libcurl/7.88.1-DEV Schannel zlib/1.2.13

Platform: Windows 10 (build 19045), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Aug 14 2023 02:42:39
	wxWidgets: 3.2.2 (wchar_t,wx containers)
	Boost: 1.81.0
	OCC: 7.7.1
	Curl: 7.88.1-DEV
	ngspice: 40
	Compiler: Visual C++ 1936 without C++ ABI

Build settings:
	KICAD_SPICE=ON