in reply to md5_hex diff input produces same output?

Although your example is flawed, there has been demonstrated working code that produces the same MD5 output:
C:\TEMP> md5sum hello.exe cdc47d670159eef60916ca03a9d4a007 C:\TEMP> .\hello.exe Hello, world! (press enter to quit) C:\TEMP> C:\TEMP> md5sum erase.exe cdc47d670159eef60916ca03a9d4a007 C:\TEMP> .\erase.exe This program is evil!!! Erasing hard drive...1Gb...2Gb... just kidding! Nothing was erased. (press enter to quit) C:\TEMP>
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