C# Dictionary issue for reference type as Key

I have class that implements interface as below

class Person : IHuman
{
}

I have created dictionary as below

Dictionary <IHuman, collection<int>> dic = new Dictionary <IHuman, collection<int>>();

Now I add one key value pair as belwow

dic.Add (person, myCollection);

Again when I use containsKey for "same person object and with same HashCode" as below

if (dic.ContainsKey(person))
{
    dic[person] = mynewcollection;
}
else
{
    dic.Add (person, mynewcollection);
}

Here ContainsKey() returns false and creates one more key value pair with same person object.

I wondered how it possible... Please help me to sort out this issue.