Q:

How to know the azure role assignment already exists

I am trying to get the current role assignment status.
e.g : I assigned contributer role to my web app
New-AzureRmRoleAssignment -ObjectId 43fddad0-xxxx-xxxx-xxxx-665033f5fbf0 -RoleDefinitionName "Storage Blob Data Contributor" -Scope "/subscriptions/4364666b-xxxx-xxxx-xxxx-47158904c439/resourceGroups/devt002RG/providers/Microsoft.Storage/storageAccounts/devt002"

When i run the above command in VSTS again it will give error message 
2019-04-02T10:22:58.2614183Z ##[error]The role assignment already exists.

I just want to get the current status of role (Storage Blob Data Contributor) assignment to my web app, if it is already exist i will skip the assignment.
I tried :
Get-AzureRmRoleAssignment -Scope "/subscriptions/4364666b-xxxx-xxxx-xxxx-47158904c439/resourceGroups/devt002RG/providers/Microsoft.Storage/storageAccounts/devt002"
Get-AzureRmRoleAssignment -Id 43fddad0-xxxx-xxxx-xxxx-665033f5fbf0

A:

well, you are using the right command, you just need to build some logic around it to be able to determine if the assignment already exists. 
If you know the GUID beforehand:
Get-AzRoleAssignment -ObjectId "GUID"

Is the easiest way to check

