MySQL 8.0

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create table yourtable (yourtext varchar(100), yourmonth varchar(100));

insert into yourtable values ('text1','2023-02');
insert into yourtable values ('text2','2023-02');
insert into yourtable values ('text3','2023-02');
insert into yourtable values ('text3','2020-10');
insert into yourtable values ('text1','2019-01');
insert into yourtable values ('text2','2020-01');
insert into yourtable values ('text3','2020-09');
SELECT * from yourtable


yourtext
yourmonth


text1
2023-02

text2
2023-02

text3
2023-02

text3
2020-10

text1
2019-01

text2
2020-01

text3
2020-09


SELECT yourmonth, SUM(is_duplicate) as duplicate_count FROM
  (SELECT t1.yourmonth, t1.yourtext, MAX(CASE WHEN t2.yourmonth IS NULL THEN 0 ELSE 1 END) AS is_duplicate
  FROM yourtable t1
  LEFT JOIN yourtable t2 ON t1.yourtext = t2.yourtext AND t1.yourmonth > t2.yourmonth
  group by t1.yourmonth, t1.yourtext) as T
GROUP BY yourmonth


yourmonth
duplicate_count


2023-02
3

2020-10
1

2019-01
0

2020-01
0

2020-09
0

yourtext	yourmonth
text1	2023-02
text2	2023-02
text3	2023-02
text3	2020-10
text1	2019-01
text2	2020-01
text3	2020-09