double defined element in parser should overwrite or give error if incompatible

[Jochem Snuverink]

currently if done this:

• d1: drift, l=0.3*m
• d1: drift, l=0.5*m

the first definition is always considered. I suggest to overwrite parameters in case the type is the same and to give an error and exit if the types are different.

[Jochem Snuverink]

duplicate of #43