Tensorflow ArgumentError: argument --model_dir: conflicting option string: --model_dir
The example you linked is from version 0.12 but the current tensorflow version is 1.2. The example underwent a lot of changes from 0.12 to 1.0. Actually, the very line you are referencing is no longer there in the last version.

This way, it's possible that you are using a version of tensorflow that is no longer compatible with the APIs used by the example. Ensure that the version of tensorflow you are using is the same as the example.