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The solution was to replace src\sys.o in the linker script with *sys.o.

As stated in the ld manual,

In any place where you may use a specific file or section name, you may also use a wildcard pattern. The linker handles wildcards much as the Unix shell does. A '*' character matches any number of characters. A '?' character matches any single character. The sequence '[chars]' will match a single instance of any of the chars; the '-' character may be used to specify a range of characters, as in '[a-z]' to match any lower case letter. A '' character may be used to quote the following character.

When a file name is matched with a wildcard, the wildcard characters will not match a '/' character (used to separate directory names on Unix). A pattern consisting of a single '*' character is an exception; it will always match any file name. In a section name, the wildcard characters will match a '/' character.

Wildcards only match files which are explicitly specified on the command line. The linker does not search directories to expand wildcards. However, if you specify a simple file name --a name with no wildcard characters-- in a linker script, and the file name is not also specified on the command line, the linker will attempt to open the file as though it appeared on the command line.

The sys.o file was specified on the command line with another path than in the linker file, so the linker tried to link it twice.