Checking for duplicates in a complex object using Linq or Lambda expression -

i've started learning linq , lambda expressions, , seem fit finding duplicates in complex object collection, i'm getting little confused , hope can put me on path happy coding.

my object structured list.list.uniquecustomeridentifier

i need ensure there no duplicate uniquecustomeridentifier in entire complex object. if there duplicates, need identify duplicated , return list of duplicates.

• unpack hierarchy
• project each element uniqueid property
• group these id's up
• filter groups groups have more 1 element
• project each group group's key (back uniqueid)
• enumerate query , store result in list.

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var result =    mylist     .selectmany(x => x.innerlist)     .select(y => y.uniquecustomeridentifier)     .groupby(id => id)     .where(g => g.skip(1).any())     .select(g => g.key)     .tolist()