title: pandas dataframe count duplicates by group and date
body: <p>I have a dataframe with 3 columns:</p>
  <ul>
  <li><code>'Date'</code> unique for each row</li>
  <li><code>'Group'</code> group to which each row belongs</li>
  <li><code>'ID'</code> identifier for each row</li>
  </ul>
  <p>In the <code>'ID'</code> columns there are some duplicate values. I want to count the number of repetitions for each group and for each date.<br />
  Example of dataframe:</p>
  <pre><code>                   Date Group    ID
  0   2021-10-29 09:15:52     B  9352
  1   2021-10-29 10:58:57     A  9352
  2   2021-10-29 11:20:46     C  9352
  3   2021-10-29 12:47:34     C  6274
  4   2021-10-29 15:41:35     C  1677
  5   2021-10-29 16:12:39     B  1677
  6   2021-10-29 18:57:56     B  9225
  7   2021-10-29 19:46:46     C  9225
  8   2021-10-30 01:23:07     C  9225
  9   2021-10-30 02:13:57     A  9225
  10  2021-10-30 05:03:52     B  9329
  11  2021-10-30 07:48:39     B  9329
  12  2021-10-30 08:45:00     A  9329
  13  2021-10-30 11:17:47     C  9329
  14  2021-10-30 21:46:07     C  9496
  15  2021-10-30 22:13:29     A  1218
  16  2021-10-31 05:39:38     C  2422
  17  2021-10-31 05:39:41     C  9654
  18  2021-10-31 10:10:21     A  1951
  19  2021-10-31 10:19:45     A  1951
  20  2021-10-31 16:40:10     A  1951
  21  2021-10-31 16:41:23     C  1951
  22  2021-10-31 22:07:16     A  1951
  23  2021-11-01 00:26:30     C  2867
  24  2021-11-01 01:25:46     B  2867
  25  2021-11-01 01:53:16     B  4262
  26  2021-11-01 01:58:30     A  4581
  27  2021-11-01 05:23:26     C  1734
  28  2021-11-01 05:38:22     C  1734
  29  2021-11-01 05:47:43     C  1734
  30  2021-11-01 06:49:27     A  4813
  31  2021-11-01 07:54:02     C  4813
  32  2021-11-01 12:10:48     C  8661
  33  2021-11-01 14:32:50     C  4138
  34  2021-11-01 18:38:16     B  4138
  35  2021-11-01 21:33:37     C  4138
  36  2021-11-02 02:34:53     B  4138
  37  2021-11-02 04:45:56     C  4138
  38  2021-11-02 07:33:38     C  4138
  39  2021-11-02 07:40:33     C  4138
  40  2021-11-02 08:06:21     B  4138
  41  2021-11-02 08:32:20     C  4138
  42  2021-11-02 09:47:26     A  4138
  43  2021-11-02 15:51:33     C  4138
  44  2021-11-02 16:04:33     B  2433
  45  2021-11-02 20:47:01     B  2433
  46  2021-11-03 04:11:57     A  4594
  47  2021-11-03 12:36:16     A  6829
  48  2021-11-03 14:42:14     A  6829
  49  2021-11-03 18:03:27     B  7138
  50  2021-11-03 18:46:46     C  7138
  51  2021-11-03 19:07:01     C  7138
  52  2021-11-03 19:23:02     A  9752
  53  2021-11-03 21:10:51     A  2699
  54  2021-11-04 00:58:12     C  2699
  55  2021-11-04 03:44:12     A  7463
  56  2021-11-04 05:40:07     C  4558
  57  2021-11-04 05:56:51     C  7855
  58  2021-11-04 06:27:28     C  7855
  59  2021-11-04 07:50:46     C  7855
  </code></pre>
  <p>Desired result:</p>
  <pre><code>          Date Group  Repetitions
  0   2021-10-29     A            1
  1   2021-10-29     B            3
  2   2021-10-29     C            3
  3   2021-10-30     A            2
  4   2021-10-30     B            2
  5   2021-10-30     C            2
  6   2021-10-31     A            4
  7   2021-10-31     B            0
  8   2021-10-31     C            1
  9   2021-11-01     A            1
  10  2021-11-01     B            2
  11  2021-11-01     C            7
  12  2021-11-02     A            1
  13  2021-11-02     B            4
  14  2021-11-02     C            5
  15  2021-11-03     A            3
  16  2021-11-03     B            1
  17  2021-11-03     C            2
  18  2021-11-04     A            0
  19  2021-11-04     B            0
  20  2021-11-04     C            4
  </code></pre>
  <p>Note that the <em>duplicate</em> conditions spans over dates and groups: the <code>'ID'</code> <code>2699</code> in the example above counts as duplicate even if those repetitions belongs to different dates and groups.</p>