Import CSV with same column name

If in a csv file multiple columns use the same name, during the import it should be checked that the columns have unique names. Otherwise it makes problems when saving and loading the project, because the first found column will be assigned to the curve and so two curves might get assigned the same column instead of differents

Release build 2.9.0-592-gafd15ba16
Sep 19 2022, 04:31:19
System: Windows 10 Version 2009
Locale: German,Germany (Decimal point ',', Group separator '.')
Number settings: Decimal point '.', Group separator ',', Exponential 'e', Zero digit '0', Percent '%', Positive/Negative sign '+'/'-' (Updated on restart)
Architecture: x86_64-little_endian-llp64
Kernel: winnt 10.0.19043
C++ Compiler: MSVC 19.29.30038.1
C++ Compiler Flags: /DWIN32 /D_WINDOWS /W3 /GR /EHsc /wd4250 /wd4251 /wd4396 /wd4661 /Zc:__cplusplus -O2 -W3 -DPSAPI_VERSION=1