Add What Are Duplicates.md

Sources/BijectiveDictionary/Documentation.docc/What Are Duplicates?.md

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+# What Are Duplicates?
+Learn what `BijectiveDictionary` means when referring to duplicates.
+
+## Duplicates
+Throughout the documentation we will use the term `duplicate`. This term has a slightly different meaning in `BijectiveDictionary` than it means in other collections such as `Dictionary`. 
+
+### Duplicates in `Dictionary`
+A `Dictionary` cannot have duplicate keys, but it can have duplicate values. Therefore when you create a `Dictionary` using the initializer `Dictionary.init(uniqueKeysWithValues:)`, you must provide unique keys. In other words the keys can have no duplicates. But it is okay for the values to have duplicates. Providing duplicate keys will result in a runtime error. 
+
+For this reason, `Dictionary` also comes with another initializer (`Dictionary.init(_:uniquingKeysWith:)`). This initializer will not result in a runtime error. In order to achieve this, you must provide it a closure which will be called any time that duplicate keys are encountered. This will instruct the initializer how to resolve the duplicate. 
+
+### Duplicates in `BijectiveDictionary`
+`BijectiveDictionary` aims to mimic `Dictionary`'s API as closely as possible, only making changes when necessary. Unlike `Dictionary`, a `BijectiveDictionary` requires both the left and the right values to be unique. But let's be a little more specific on what that means. 
+
+1. Within the left values, there can be no duplicates. 
+2. Within the right values, there can be no duplicates. 
+3. However, it is okay if the same value exists on both the right and left side. This is not considered a duplicate.