There is already an object named in the database

it seems there is a problem in migration process, run add-migration command in "Package Manager Console":

Add-Migration Initial -IgnoreChanges

do some changes, and then update database from "Initial" file:

Update-Database -verbose

Edit: -IgnoreChanges is in EF6 but not in EF Core, here's a workaround: https://stackoverflow.com/a/43687656/495455

Maybe you have changed the namespace in your project!
There is a table in your data base called dbo.__MigrationHistory. The table has a column called ContextKey.
The value of this column is based on your namespace. for example is "DataAccess.Migrations.Configuration".
When you change the namespace, it causes duplicate table names with different namespaces.
So, after you change namespace in code side, change the namespace in this table in database, too, (for all rows).
For example, if you change the namespace to EFDataAccess, then you should change the values of ContextKey column in dbo.__MigrationHistory to "EFDataAccess.Migrations.Configuration".
Then in code side, in Tools => Package Manager Console, use the update-database command.

Another option instead of changing the context value in the database is to hard code the context value in your code to the old namespace value. This is possible by inheriting DbMigrationsConfiguration<YourDbContext> and in the constructor just assign the old context value to ContextKey, than inherit from MigrateDatabaseToLatestVersion<YourDbContext, YourDbMigrationConfiguration> and leave that class empty. The last thing to do is call Database.SetInitializer(new YourDbInitializer()); in your DbContext in a static constructor.

I hope your problem will be fixed.

"There is already an object named 'AboutUs' in the database."

This exception tells you that somebody has added an object named 'AboutUs' to the database already.

AutomaticMigrationsEnabled = true; can lead to it since data base versions are not controlled by you in this case. In order to avoid unpredictable migrations and make sure that every developer on the team works with the same data base structure I suggest you set AutomaticMigrationsEnabled = false;.

Automatic migrations and Coded migrations can live alongside if you are very careful and the only one developer on a project.

There is a quote from Automatic Code First Migrations post on Data Developer Center:

Automatic Migrations allows you to use Code First Migrations without having a code file in your project for each change you make. Not all changes can be applied automatically - for example column renames require the use of a code-based migration.

Recommendation for Team Environments

You can intersperse automatic and code-based migrations but this is not recommended in team development scenarios. If you are part of a team of developers that use source control you should either use purely automatic migrations or purely code-based migrations. Given the limitations of automatic migrations we recommend using code-based migrations in team environments.

In my case, my EFMigrationsHistory table was emptied (somehow) and when trying to run update-database I would get:

There is already an object named 'AspNetUsers' in the database

After seeing the table had been emptied it made sense that it was trying to rerun the initial migration and trying to recreate the tables.

To fix this problem I added rows into my EFMigrationsHistory table. 1 row for each migration that I knew the database was up to date with.

A row will have 2 columns: MigrationId and ProductVersion

MigrationId is the name of your migration file. Example: 20170628112345_Initial

ProductVersion is the ef version you're running. You can find this by typing Get-Package into the Package Manager Console and looking for your ef package.

Hope this is helpful for someone.