There’s no conceptual duality of: [Value] vs [Exchange-Value]

I agree with you that there is no duality, but there is a distinction.

I think you are referring to this bit from chapter 1?:

“When, at the beginning of this chapter, we said, in common parlance, that a commodity is both a use value and an exchange value, we were, accurately speaking, wrong. A commodity is a use value or object of utility, and a value. It manifests itself as this two-fold thing, that it is, as soon as its value assumes an independent form – viz., the form of exchange value. It never assumes this form when isolated, but only when placed in a value or exchange relation with another commodity of a different kind. When once we know this, such a mode of expression does no harm; it simply serves as an abbreviation.”

Exchange-value is a value-form or form of value. The difference is that use-value/value are an internal opposition whereas use-value/exchange-value are an external opposition, because in the latter case they are mere forms of appearance of an internal content.

It sounds like a pedantic point, but distinction between form and content occurs in other places in Capital. Value takes on various forms through the productive process, and these forms are important to study, not just value in the abstract. Money as a form of value has peculiar features and functions that distinguish it from commodities, which are themselves forms of value. And yet was also important for Marx to unify the various forms of surplus value under one general theory of surplus value; he wrote to Engels that one of the best points in Capital is “the treatment of surplus value independently of its particular forms as profits, interest, ground rent, etc.”

And while value is a content which takes various forms, value is itself a form, whose content is social abstract labor; or in other words, value is a form of the social relations of commodity production.

Sections 4 and 5 in the appendix on the value-form elaborate a bit on the distinction between exchange-value and value:

§ 4. As soon as value appears independently it has the form of exchange-value

The expression of value has two poles, relative value-form and equivalent-form. To start with, what concerns the commodity functioning as equivalent is that it counts for another commodity as the shape of value (Wertgestalt), a body in immediately exchangeable form – exchange-value. But the commodity whose value is expressed relatively, possesses the form of exchange-value in that:

1. its existence as value is revealed by the exchangeability of the body of another commodity with it;
2. its magnitude of value is expressed through the proportion in which the other commodity is exchangeable with it.

The exchange-value is hence the independent form of appearance of commodity-value.

§5. The simple value-form of the commodity is the simple form of appearance of the opposites, use-value and exchange-value contained within it

In the relation of value of the linen to the coat the natural form (Naturalform) of the linen counts only as the shape (als Gestalt) of use-value, the natural form of the coat only as value-form (Wertform) or shape (Gestalt) of exchange-value. The inner opposition between use-value and value (Gebrauchswert und Wert) contained in a commodity is thus represented by an external opposition, i.e. the relation of two commodities, of which the one counts immediately only as use-value, the other immediately only as exchange-value, or in which the two opposing determinations, use-value and exchange-value, are distributed in a polar manner among the commodities.

If I say: As a commodity the linen is use-value and exchange-value, this is my judgement about the nature of the commodity gained by analysis. As opposed to this, in the expression ‘20 yards of linen = 1 coat’ or ‘20 yards of linen are worth 1 coat’ the linen itself says that it

1. is a use-value (linen);
2. is an exchange-value distinct from that (something equal to the coat); and
3. is the unity of these two differences, and thus is a commodity.