find duplicates in list python

def duplicate_list_elements(_list : list):
    duplicated_list = []

    for element in _list:
        duplicated_list.append(element)
        duplicated_list.append(element)

    return duplicated_list

a = [1,2,3,2,1,5,6,5,5,5]

import collections
print([item for item, count in collections.Counter(a).items() if count > 1])

## [1, 2, 5]

def Repeat(x): 
    _size = len(x) 
    repeated = [] 
    for i in range(_size): 
        k = i + 1
        for j in range(k, _size): 
            if x[i] == x[j] and x[i] not in repeated: 
                repeated.append(x[i]) 
    return repeated 
  
list1 = [10, 20, 30, 20, 20, 30, 40,  
         50, -20, 60, 60, -20, -20] 
print (Repeat(list1)) 

#output:[20, 30, -20, 60]

print()
    
def Find_Repeated(x):
    x2=sorted(x)
    List_Of_Repeated=[]
    for i in x2:
        if x2.count(i)>1:
            List_Of_Repeated.append([i,x2.count(i)])
            for c in range(x2.count(i)):
                for j in x2:
                    if i==j and x2.count(i)>1:
                        x2.remove(i)
    List_Of_Repeated.sort()
    return List_Of_Repeated

List=[1,2,3,4,4,4,5,5,5,5,5,1,1,2,2,3,7,8,6]

# Repeated numbers: [1,2,3,4,5]

# Simple print output:

print(Find_Repeated(List),"\n")

# For a neat output:

print("[ Value , Times Repeated ] \n")
print("For example: [2,4]  The value 2 was repeated 4 times. \n")
for i in Find_Repeated(List):
    print(i)

print()