Handle duplicate columns when merging input files

Given this command:

funpack out.tsv one.tsv two.tsv

if one.tsv and two.tsv both have a column with the same name, funpack will crash during the import stage:

Traceback (most recent call last):                                                                    
  File "/opt/fmrib/fsl/bin/funpack", line 10, in <module>                                             
    sys.exit(main())
  File "/opt/fmrib/fsltmp/fsl_59f8789b/fslpython/envs/fslpython/lib/python3.7/site-packages/funpack/mai
n.py", line 124, in main                                                                              
    dtable, unknowns, uncategorised, drop = doImport(args, mgr)                                       
  File "/opt/fmrib/fsltmp/fsl_59f8789b/fslpython/envs/fslpython/lib/python3.7/site-packages/funpack/mai
n.py", line 209, in doImport                                                                          
    dryrun=args.dry_run)                                                                              
  File "/opt/fmrib/fsltmp/fsl_59f8789b/fslpython/envs/fslpython/lib/python3.7/site-packages/funpack/imp
orting/core.py", line 196, in importData                                                              
    dryrun=dryrun)
  File "/opt/fmrib/fsltmp/fsl_59f8789b/fslpython/envs/fslpython/lib/python3.7/site-packages/funpack/imp
orting/core.py", line 364, in loadFiles
    pool=pool)                                                                                        
  File "/opt/fmrib/fsltmp/fsl_59f8789b/fslpython/envs/fslpython/lib/python3.7/site-packages/funpack/imp
orting/core.py", line 570, in loadFile                                                                
    chunks = pool.starmap(func, offsets)
  File "/opt/fmrib/fsltmp/fsl_59f8789b/fslpython/envs/fslpython/lib/python3.7/multiprocessing/pool.py",
 line 276, in starmap                                                                                 
    return self._map_async(func, iterable, starmapstar, chunksize).get()                              
  File "/opt/fmrib/fsltmp/fsl_59f8789b/fslpython/envs/fslpython/lib/python3.7/multiprocessing/pool.py",
 line 657, in get                                                                                     
    raise self._value                                                                                 
ValueError: Duplicate names are not allowed. 

Maybe the default behaviour should be to drop the column with less rows, and emit a warning to that effect.