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Adam JohnsonAdam Johnson 

Create a validation rule that does not allow me to generate 2 activities of the same type.

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Dharmendra ShekhawatDharmendra Shekhawat

You can do this by creating a unique field + workflow rule + field update combo. Here's a blog post that takes you through it step by step:

Remember that you will need to unique fields and two workflow rules +field updates for both Task and Event objects.If you need to check for uniqueness across the whole of Activities combined (Tasks and Events), only an Apex trigger can do this.