With identical key and IV, you'll produce identical ciphertext for identical plaintext.
Scenario: You send messages to your broker via this scheme, and don't worry about the messages being intercepted because you've encrypted them. C1 reads:
Stock: Cocoa-Cola Order: Buy Shares: 1,000,000 Price: > $20
Someone steals your SD card while you're not looking, and copies the ciphertext. Later on through other means, finds out you bought 100 shares of coca-cola.
Coke keeps going up, so you buy more shares. C2 reads:
Stock: Coca-Cola Order: Buy Shares: 3,000,000 Price: >$30
The attacker has noticed the first part of your message hasn't changed. Later on, you're observed buying 300 shares of coca-cola. Now the attacker has an idea of the format of your messages.
Coke is skyrocketing, so you decide to buy even MORE coke shares. C3 reads
Stock: Coca-Cola Order: Buy Shares: 5,000,000 Price: >$40.
The first part of the message is STILL the same, so the attacker has a damn good idea that you're going to buy Coke shares. He doesn't know how many, but based on past purchased he can have a good idea.
Thus your encryption has been foiled. Further messages back and forth will only break more and more of the ciphertext. This is similar to the scenario the NSA used against the Soviet union when the soviets re-used a one time pad in project verona https://en.wikipedia.org/wiki/Venona_project