When I try to open a new account I get the message "You have existing Interactive Brokers account(s)".

OPEN AN ADDITIONAL ACCOUNT

If you already have an existing Interactive Brokers account and want to open an additional account, please use the button above or follow the procedure below to open a linked account:…

You can link your multiple accounts under a single username and password, including accounts that are managed by an Advisor or a Broker. 

Observe these guidelines when linking accounts:…

LINK EXISTING ACCOUNTS

If you do not see the option to"Create New Individual, Joint or IRA Account" in Client Portal, it is because the system detects that there are existing accounts eligible to be linked under the same username and password. You must first link them together before you will be eligible to create a new linked account:…

Each IBKR account is only allowed one paper trading account. If you have multiple IBKR accounts, each one can have its own paper trading account.

To open a second paper trading account:…