I must somehow have messed up my phobos.lib - i don't know how!
The following code:
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module test;

private import std.stream;

int main(char[][] args)
{
    Stream stream = new BufferedFile("encodedfile.d", FileMode.OutNew);
    stream.close();

    return 0;
}
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Seems legal but gives me a linker error that i don't understand at all:
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OPTLINK (R) for Win32  Release 7.50B1
Copyright (C) Digital Mars 1989 - 2001  All Rights Reserved

C:\D\bin\..\lib\phobos.lib(ti_double)  Offset 1C8BEH Record Type 0091
 Error 1: Previous Definition Different : __init_10TypeInfo_d
C:\D\bin\..\lib\phobos.lib(ti_double)  Offset 1C8D8H Record Type 0091
 Error 1: Previous Definition Different : __Class_10TypeInfo_d
C:\D\bin\..\lib\phobos.lib(ti_double)  Offset 1C8F1H Record Type 0091
 Error 1: Previous Definition Different : __vtbl_10TypeInfo_d
---------------------------------------

Can someone explain this to me?

I upgraded from build v3.01 to v3.02
Now the problem is gone. Sorry for not trying this before i posted.