Page numbers from Fowkes penguin translation, emphasis is mine

Let us now look at the residue of the products of labour. There is nothing left of them in each case but the same phantom-like objectivity; they are merely congealed quantities of homogeneous human labour, i.e. of human labour-power expended without regard to the form of its expenditure. All these things now tell us is that human labour-power has been expended to produce them, human labour is accumulated in them. *As crystals of this social substance, which is common to them all, they are values - commodity values. *(128)

We have seen that when commodities are in the relation of exchange, their exchange-value manifests itself as something totally independent of their use-value. But if we abstract from their use-value, there remains their value, as it has just been defined. The common factor in the exchange relation, or in the exchange-value of the commodity, is therefore its value. The progress of the investigation will lead us back to exchange-value as the necessary mode of expression, or form of appearance, of value. For the present, however, we must consider the nature of value independently of its form of appearance. (128)

A commodity is a use-value or object of utility, and a 'value'. It appears as the twofold thing it really is as soon as its value possesses its own particular form of manifestation, which is distinct from its natural form. This form of manifestation is exchange-value, and the commodity never has this form when looked at in isolation, but only when it is in a value-relation or an exchange relation with a second commodity of a different kind. Once we know this, our manner of speaking does no harm; it serves, rather, as an abbreviation. (152)

But although price, being the exponent of the magnitude of a commodity's value, is the exponent of its exchange-ratio with money, it does not follow that the exponent of this exchange-ratio is necessarily the exponent of the magnitude of the commodity's value. ... The possibility, therefore, of a quantitative incongruity between price and magnitude of value, i.e. the possibility that the price may diverge from the magnitude of value, is inherent in the price-form itself. *This is not a defect, but, on the contrary, it makes this form the adequate one for a mode of production whose laws can only assert themselves as blindly operating averages between constant irregularities. *(197)

It isn't a duality, it's a series of reflections (commodity-value -> exchange-value -> price). Commodity-value is the socially necessary labour in a commodity, exchange-value is it's reflection in another commodity and price is that exchange value in money. All three of them can differ from each other to some extent (particularly 'commodity-value/value', which can never be actually observed or measured except as seen in its reflection/likeness, exchange-value and the ). All three of these things' ability to diverge from each other is necessary for the capitalist system to function on its laws of averages. Exchange-value is the equivalent of a commodity's commodity-value in another commodity (e.g. the value of a shoe in ounces of gold). Price is a representation of the ratio in which a commodity can be exchanged for money.

Also very important to note (for feminism/ecological/ableism/etc critique) that Marx is laying out the standards of capitalist value, which doesn't necessarily represent the actual work/labour exerted on a given thing, e.g. doesn't represent the labour required to produce a forest cut down.

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.