def acquisitions_from_ifg_dates(ifg_dates):
    """Given a list of ifg dates in the form YYYYMMDD_YYYYMMDD, get the uniqu YYYYMMDDs that acquisitions were made on.  
    Inputs:
        ifg_dates | list | of strings in form YYYYMMDD_YYYYMMDD.  Called imdates in LiCSBAS nomenclature.  
    Returns:
        acq_dates | list | of strings in form YYYYMMDD
    History:
        2021_04_12 | MEG | Written
        
    """
    
    acq_dates = []
    for ifg_date in ifg_dates:                                          # loop through the dates for each ifg
        dates = ifg_date.split('_')                                     # split into two YYYYMMDDs
        for date in dates:                                              # loop through each of these
            if date not in acq_dates:                                   # if it't not already in the list...
                acq_dates.append(date)                                  # add to it
    return acq_dates