Knowing which foot his partner's weight is on, and that when his upper body moves forward it will clear a space for his foot, the "Tango Man" walks assertively and confidently straight ahead. (Artistic License invoked)
You can practice the mechanics of not "duck walking" on your own. Assume your best tango posture - Alicia Pons should have given you some good information on that - and walk forward: step collect, step collect. Then step and continue to walk as your ankles brush past each other, and repeat. Make sure that you start each motion with your upper body, then move your foot forward.
Your previous posts indicate that you are interested in close embrace, so work on stepping under yourself once your upper body moves forward. Also work on being able to stop your forward momentum with each step. In close embrace as taught by Susanna Miller and Alicia Pons, you should be sharing weight with your partner, so keep that weight forward!. It's much more rewarding with a partner, but you can practice walking on your own.
Early on, when I was learning an open style of tango, actually brushing the ankles, and collecting my feet - ankles pretty much touching - was emphasized. Susanna Miller, who teaches close embrace milonguero loved that (HA!). Let's just say she encouraged me to stand with my feet a few inches apart when doing close embrace.
The more you practice walking and standing with your feet close together, the more it will become something you just do without thinking. If there are mirrors where you dance, check yourself occasionally.
Also...
When dancing or practicing with a partner, your partner plays a big part in how confidently you can walk straight ahead. You must know where her weight is before you step forward into her! One of her roles in tango is to clearly communicate to you where her weight is. If you don't know where her weight is, you don't know where you can step. You must be confident, however, that the foot she doesn't have weight on will move if your upper body goes first. If you don't know which of your partner's feet is free to move, lead a weight change or two so that you do know. It's easier with a skilled partner.