How do I find duplicates across multiple columns?

 SELECT name, city, count(*) as qty 
 FROM stuff 
 GROUP BY name, city HAVING count(*)> 1

Something like this will do the trick. Don't know about performance, so do make some tests.

select
  id, name, city
from
  [stuff] s
where
1 < (select count(*) from [stuff] i where i.city = s.city and i.name = s.name)

Using count(*) over(partition by...) provides a simple and efficient means to locate unwanted repetition, whilst also list all affected rows and all wanted columns:

SELECT
    t.*
FROM (
    SELECT
        s.*
      , COUNT(*) OVER (PARTITION BY s.name, s.city) AS qty
    FROM stuff s
    ) t
WHERE t.qty > 1
ORDER BY t.name, t.city

While most recent RDBMS versions support count(*) over(partition by...) MySQL V 8.0 introduced "window functions", as seen below (in MySQL 8.0)

CREATE TABLE stuff(
   id   INTEGER  NOT NULL
  ,name VARCHAR(60) NOT NULL
  ,city VARCHAR(60) NOT NULL
);

INSERT INTO stuff(id,name,city) VALUES 
  (904834,'jim','London')
, (904835,'jim','London')
, (90145,'Fred','Paris')
, (90132,'Fred','Paris')
, (90133,'Fred','Paris')

, (923457,'Barney','New York') # not expected in result
;

SELECT
    t.*
FROM (
    SELECT
        s.*
      , COUNT(*) OVER (PARTITION BY s.name, s.city) AS qty
    FROM stuff s
    ) t
WHERE t.qty > 1
ORDER BY t.name, t.city

    id | name | city   | qty
-----: | :--- | :----- | --:
 90145 | Fred | Paris  |   3
 90132 | Fred | Paris  |   3
 90133 | Fred | Paris  |   3
904834 | jim  | London |   2
904835 | jim  | London |   2

db<>fiddle here

Window functions. MySQL now supports window functions that, for each row from a query, perform a calculation using rows related to that row. These include functions such as RANK(), LAG(), and NTILE(). In addition, several existing aggregate functions now can be used as window functions; for example, SUM() and AVG(). For more information, see Section 12.21, “Window Functions”.

I found this way to be pretty flexible / efficient

select 
    s1.id
    ,s1.name
    ,s1.city 
from 
    stuff s1
    ,stuff s2
Where
    s1.id <> s2.id
    and s1.name = s2.name
    and s1.city = s2.city

SELECT Feild1, Feild2, COUNT(*)
FROM table name
GROUP BY Feild1, Feild2
HAVING COUNT(*)>1

This will give you all your answers.

You have to self join stuff and match name and city. Then group by count.

select 
   s.id, s.name, s.city 
from stuff s join stuff p ON (
   s.name = p.city OR s.city = p.name
)
group by s.name having count(s.name) > 1

From OP question, OP wants to group columns and get additional columns that aren't grouping columns.

so that regular group by + having might not be worked.

I would use EXISTS subquery with HAVING.

we can try to add columns which you want to mark duplicate in a subquery.

SELECT s.id, s.name,s.city 
FROM stuff s
WHERE EXISTS (
   SELECT 1
   FROM stuff ss
   WHERE 
      s.name = ss.name
   AND
      s.city = ss.city
   GROUP BY ss.name,ss.city
   HAVING COUNT(*) > 1
)

If we create a suitable Index might get better performance than join

CREATE INDEX IX_name ON stuff (
    name,
    city
);

Another way we can use COUNT window function with filter condition to make it which add grouping columns in PARTITION BY part

SELECT s.id, s.name,s.city 
FROM (
   SELECT *,COUNT(*) OVER(PARTITION BY name,city) cnt
   FROM stuff 
) s
WHERE cnt > 1

sqlfiddle

It's a pleasure to add another way of achieving the required output using Cross Apply here like below :

select s.* from stuff s
cross apply(
    select name, city from stuff
    group by name, city
    having Count(*) > 1) x
where s.name = x.name and s.city=x.city

Given a staging table with 70 columns and only 4 representing duplicates, this code will return the offending columns:

SELECT 
    COUNT(*)
    ,LTRIM(RTRIM(S.TransactionDate)) 
    ,LTRIM(RTRIM(S.TransactionTime))
    ,LTRIM(RTRIM(S.TransactionTicketNumber)) 
    ,LTRIM(RTRIM(GrossCost)) 
FROM Staging.dbo.Stage S
GROUP BY 
    LTRIM(RTRIM(S.TransactionDate)) 
    ,LTRIM(RTRIM(S.TransactionTime))
    ,LTRIM(RTRIM(S.TransactionTicketNumber)) 
    ,LTRIM(RTRIM(GrossCost)) 
HAVING COUNT(*) > 1

.