php - select duplicate records where one record must contain the exact value -

i have table below 3 columns amount, frequency & identifier. these records in same table in same database.

amount     frequency       identifier 4.75        1                100 4.75        3                101 4.76        2                102 4.76        3                103 4.44        1                104 4.43        1                105 4.75        2                106 4.67        2                107 4.75        3                108 4.64        2                109 4.64        3                110 4.65        4                111 

i'm after sql query should pickup duplicate records different frequency frequency of 1 of them must equal 1. output i'm expecting

amount  frequency   identifier 4.75        1       100 4.75        3       101 4.75        2       106 4.75        3       108 

note query executed every 2 seconds along 15 other queries lightweight. can processing in php too, prefer database query long not performance intensive. i'm trying different queries, can't consistent results. can please ?

many thanks

i'm thinking fastest way want:

select * t exists (select 1 t t2 t2.amount = t.amount , t2.frequency = 1) 

this run fastest index on (amount, frequency).