Find duplicates across multiple columns

My spreadsheet contains information about companies and is structured ledger-like in this way:

Company   Quarter   Revenues   EBITDAA         2010.1    5          1A         2010.2    6          1B         2010.1    10         2B         2010.2    12         3B         2010.3    20         5A         2010.3    8          2B         2011.1    14         3A         2010.3    8          2A         2010.4    7          2A         2011.1    9          1

I want:

• a formula (not GAS) that will show me a list of duplicates for the combination Company-Quarter. In the above example, there are two entries for Company A for Quarter 2010.3. In the data there could be multiple duplicates.

• a formula (not GAS) that give me missing Company-quarters for a given quarters range for all companies that have at least one entry in the date range (or alternatively, from a pre-determined list of Companies). In the above example, for the range 2010.1 to 2011.1, there is no entry for Company B in 2010.4. In the data there could be multiple missing information.

Ideally, I won't need to create auxiliary columns and is all contained in one formula, so as to make the spreadsheet more elegant.