"I was seeing" is correct and can be used in written English. You can use it to indicate that something happened in the past. For example, "I was seeing double after the long day at work".
I was seeing the line well.
Perhaps I was seeing things.
I couldn't believe I was seeing it.
I was seeing that it doesn't work".
"I was seeing stripes everywhere," he said.
I hinted that I was seeing someone.
What I was seeing was utterly vulnerable".
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Ha Thuy Vy
MA of Applied Linguistic, Maquarie University, Australia