[SalesForce] Get Unique keys map and Repeated keys map

Map of lists maybe?

List<Account> fakeQuotes = new List<Account>{
    new Account(Name = 'Prod 1', AnnualRevenue = 100),
    new Account(Name = 'Prod 2', AnnualRevenue = 200),
    new Account(Name = 'Prod 1', AnnualRevenue = 300),
    new Account(Name = 'Prod 2', AnnualRevenue = 400),
    new Account(Name = 'Prod 1', AnnualRevenue = 500)
};

Map<String, List<Double>> listPrices = new Map<String, List<Double>>();
for(Account a : fakeQuotes){
    List<Double> temp = listPrices.get(a.Name);
    if(temp == null) {
        listPrices.put(a.Name, new List<Double>{a.AnnualRevenue});
    } else {
        temp.add(a.AnnualRevenue);
    }
}

System.debug(listPrices); // {Prod 1=(100.0, 300.0, 500.0), Prod 2=(200.0, 400.0)}

I've come across a similar issue before in Using an sObject as a Map key.

From Map Considerations:

Uniqueness of map keys of user-defined types is determined by the equals and hashCode methods, which you provide in your classes. Uniqueness of keys of all other non-primitive types, such as sObject keys, is determined by comparing the objects’ field values.

And sObject Map Considerations

Be cautious when using sObjects as map keys. Key matching for sObjects is based on the comparison of all sObject field values. If one or more field values change after adding an sObject to the map, attempting to retrieve this sObject from the map returns null. This is because the modified sObject isn’t found in the map due to different field values. This can occur if you explicitly change a field on the sObject, or if the sObject fields are implicitly changed by the system; for example, after inserting an sObject, the sObject variable has the ID field autofilled. Attempting to fetch this Object from a map to which it was added before the insert operation won’t yield the map entry, as shown in this example.

Something interesting I found. As you say, the assertions pass when I run your sample code as anonymous apex with the Apex Code logging level set to DEBUG. However, with exactly the same code, if you set the Apex Code logging level to FINEST it will fail the assertion!